tl;dr

• The best buddy CP boost gives an advantage of a similar magnitude to having better IVs.
• Because typing and moveset is more important than IVs, relatively few Pokemon will get a major boost. Most are overshadowed by their evolutions which have better movesets (e.g. Bayleef, Kabuto).
• Farfetch'd, Sableye, Medicham, Dustox, Magnemite, Grimer, Yanma and Nidorino are some Pokemon that may benefit, although I'd only really recommend the first four.
• Breakpoints still matter. A boosted Sableye with optimised IVs loses matchups against Registeel that an optimised unboosted Sableye can beat.

When I first saw the news that best buddies had a 5 level CP boost, I got very excited. Given its former overwhelming dominance at 1431CP, what could a 1500CP Medicham do?

We've seen some great analysis from u/speedyjunk11 about the improvement in stat product for various Pokemon that sit near the 1500 and 2500 CP thresholds. The biggest improvement in Great League was Kabuto, which gets a 9.9% boost to its stat product. The average improvement of the listed Pokemon was 5.3%. By way of comparison, a Skarmory with 15/0/0 IVs has a stat product which is 9.38% less than the stat product of a rank 1 Skarmory.

So to answer the general question, how important are best buddies, they are as important as having good IVs. Kieng's oft quoted adage holds true for both IVs and best buddies. IVs matter, but they are like the 10th most important thing. What's more important are things like typing and moveset. With that said, let's look at the Pokemon I think will benefit most from the buddy CP boost.

Sableye (Shadow Claw Foul Play/Power Gem)

Level 40: 1476CP 123.2 Attack, 119.3 Defence, 120 Stamina with 15/15/15 IVs

Level 45: 1500CP 112.2 Attack, 122.2 Defence, 123 Stamina with 9/14/14 optimised IVs

Sableye would seem the optimal candidate for the buddy boost. Already a popular pick in freestyle, it's best known as a safe swap. It doesn't have many crushing victories, but it also doesn't have many crushing defeats. So surely a CP boost would flip some of those soft losses into soft wins, right?

I ran sims on pvpoke comparing the performance of a level 40 and level 45 Sableye in even shield scenarios (no shields, 1 shield with best case shielding (1BC), 1 shield with worse case shielding (1WC)). Both Sableye and its opponents had IVs that maximised their stat product.

The boosted Sableye does perform better. Against 16 core freestyle pokemon, it has an average battle rating of 510 compared to 489 for its unloved counterpart. In the extended meta, 45Sableye has an average battle rating of 531 compared to an average of 517 for 40Sableye.

But, it has the same number of wins against the core meta. While 45Sableye wins the mirror, optimised 45Sableye loses the matchup against Registeel. That's because there's a breakpoint allowing shadow claw to deal 4 damage to Registeel, requiring 122.92 attack. Unfortunately, with its slightly lower attack, optimised 45Sableye just misses out against an optimised Registeel and gets hit by a third Flash Cannon. Of course, a 45Sableye with different stats (e.g. 10/13/13) can beat an optimised Registeel. So just as a rank 1 pokemon can lose match-ups that lesser ranks can win, sometimes you don't want an optimised best buddy. But good luck predicting those breakpoints 30 days in advance (or 15 days if you want to burn coins on those poffins!)

Against the extended meta, 45Sableye flips hard losses against Victreebel (324 BR) in the 1BC and 1WC scenarios to soft wins (508 BR). It flips a 1BC tie/1WC loss (319 BR) against Vigoroth to soft wins of 504 and 520 BR respectively. It also flips 1BC and 1WC ties against Froslass into soft wins. In the core meta, it also flips a soft loss against Meganium (406 BR) in a no shield scenario to a tie.

Conclusion: If you were already planning on using a Sableye, then the buddy boost may be worth the effort. But the new wins just aren't that consequential.

Medicham (Counter Dynamic Punch/Ice Punch)

An old school freestyle champion that's fallen out of favour recently with the PUP nerf, Medicham is the other obvious candidate. I'm simming a Medicham with Dynamic Punch and Ice Punch, rather than the classic PUP/Ice Punch combination.

Again, 45Medicham performs better across the board with an average BR against the core meta of 532.5 and 489.6 against the extended meta. Its unloved 40Medicham cousin has averages of 514.3 and 467.9. And again, we see that relatively few losses flip over into wins.

Against the core meta, 45Medicham wins the mirror but the only other win is against WG Lanturn in a no shield contest (so Medicham now beats Lanturn across all shields).

In the extended meta, there are a few consequential flips. The most important is flipping the very soft losses against Toxicroak in 1BC and 1WC into very soft wins. Similarly, it flips ties against Victreebel in 1BC and 1WC into soft wins. The previously iffy match-up against Jirachi (1BC win of 682, 1WC loss of 383) is now a slightly less iffy 1BC win (717BR), 1WC tie. It also turns a 0s loss against Alolan Muk into a comfortable win.

Conclusion: 45Medicham is legitimately stronger than 40Medicham. Aside from Gallade, Medicham beats all other fighters (including Deoxys). But its handful of new wins won't catapult it back to its old glory.

Bastiodon, Azumarill

I haven't simmed it, but 45Bastiodon gets a small 2.77% boost to its stat product and 45Azu gets a 2.87% boost. Could flip a few wins.

Conclusion: Probably only worth it if you found a top ranked 45Azu/Bast or you don't have a highly ranked 40Azu/Bast.

Dustox (Confusion Sludge Bomb/Silver Wind)

A popular alternative to legacy Venomoth, boosted Dustox could prove to be even more popular. While unboosted Dustox had a lower stat product than Venomoth, a boosted Dustox now has a higher stat product.

Venomoth still has - arguably - the better moveset (Confusion Poison Fang/Silver Wind). You already have a bazooka charge move in Silver Wind (84 damage after 8 seconds), you generally don't need a second bazooka or nuke in the form of Sludge Bomb (96 damage, 10 seconds). You'd rather have legacy Venomoth's spammy poison fang (48 damage, 6 seconds).

Conclusion: Totally worth it if you don't have a legacy Venomoth. And possibly worth it even if you have it due to its marginally better stats.

Magnemite (Thunder Shock Discharge or Thunderbolt/Magnet Bomb)

For those of you without a double legacy Magneton (Thunder Shock Discharge/Magnet Bomb), rejoice! 45Magnemite is your glassier alternative. Magnemite has stats of 147/111/88 compared to Magneton's 141/116/96.

You can use the same moveset as Magneton, or swap discharge (charges every 5 secs for 65 damage) for thunderbolt (charges every 7 secs dealing 90 damage).

Conclusion: Totally worth it if you don't have a legacy Magneton

Farfetch'd (Fury cutter leaf blade/air cutter or aerial ace)

With u/JRE47 muttering about Farfetch'd and its potential in Fusion Cup (or Timeless, not sure which), it was a silly omission for me to leave out this peaking duck (a poor pun as my homage to my fellow writer). Thanks to u/Bendixy for the tip!

I've never looked at Farfetch'd before, but I can see its potential with a high energy fast move and one of the highest DPE moves, leaf blade. It spits out leaf blades with 70 damage every 4.5 seconds (the second fastest available timing). You probably don't need a second charge move except for coverage. Aerial ace does 66 damage every 6 secs and air cutter deals 72 damage every 7 seconds.

Farfetch'd also gains a 9.9% boost to its stat product! This is bigger on anything in u/speedyjunk11's spreadsheet. But with a CP of 1315, 45Farfetch'd's stats may keep it from being a core pick for future metas. It is, however, likely to become a legitimate off-meta pick in future cups.

Conclusion: With its spammy, high damage leaf blades, Farfetch'd's time as a niche pick could come to an end.

Yanma (Wing Attack Ancient Power/Silver Wind/Aerial Ace)

45Yanma has marginally better stats than its evolution (but IVs could make the difference here), Yanmega but lacks a hard hitting charge move. Yanmega has all the same moves as Yanma, but it also has Bug Buzz as an alternative charge move hitting for 108 damage (after STAB).

Conclusion: All in all, it hardly seems the effort and stardust to buddy boost a level 40 Yanma when you could just evolve a Yanmega much more quickly.

Grimer (Poison Jab Sludge Bomb/Mud bomb)

Sporting a high energy fast move and an excellent combo of a spammy mud bomb and hard hitting sludge bom, Kanto Grimer has been an off-meta pick for a few cups. Best buddy status can increase its stat product by 9.71%, making Grimer one of the winners from this new mechanic.

Despite its upgrade, Grimer still lies in the shadow of its evolved form. Muk has a wider array of charge moves, including dark pulse and thunder punch. Muk's thunder punch charges in 6 seconds - the same time it takes Grimer to charge its mud bomb. However, Muk's poison charge moves (sludge wave, 10 secs and gunk shot, 11 secs) act as more of a nuclear option when sometimes a bazooka (Grimer's sludge bomb, 8 secs) will do.

Conclusion: Although generally overshadowed by Muk's superior stats and wider movepool, Grimer could carve out a niche where you'd rather a spammy ground move than a spammy electric move.

Nidorino (Poison Jab Sludge Bomb/Horn Attack)

Nidorino isn't exactly a super star for the same reasons, poison jab Nidoking isn't a super star. But with a different typing (pure poison v poison ground), access to sludge bomb and a spammy horn attack, Nidorino could see niche play in certain formats.

Conclusion: I struggle to see a meta where Nidorino is key. But who really knows!

Kabuto (Mud shot Ancient Power/Aqua Jet) - not recommended

While I mentioned Kabuto earlier as the Pokemon with the biggest boost to its stat product, it is held back by its moveset and is overshadowed by its evolution. It lacks a hard hitting charge move that can really take advantage of its high energy fast move. Ancient power is a good move - dealing 84 STAB-adjusted damage every 5 seconds - but it doesn't seem quite enough to catapult Kabuto into the core meta of any future cups. And if it did become meta, Kabutops has mudshot ancient power, but it also has access to stone edge and has a higher stat product.

Delcatty (Charm Wild Charge/Disarming Voice) - not recommended

45Delcatty is a decent charmer. It gets a small 3.32% stat boost, but this gives it a higher stat product than Togekiss, Clefable and Wigglytuff. But without the extra STAB damage for charm and very slow charge moves (the fastest, disarming voice takes 12 seconds), it's just not worth it.

Maybe feint attack as a fast attack could be useful in certain metas.

Other not recommended pokemon

Similar analysis could be applied to Pokemon like Bayleef and Poliwhirl. All are generally beaten by their evolved brethren either in terms of stat product or movesets. That isn't to say they couldn't find a use - Bayleef may be preferable to using razor leaf Meganium because it has access to ancient power, but in most cases the unholy damage caused by Vine Whip/Frenzy Plant Meganium outweighs any benefits.

While Exeggcute was useful in past cups, I think that was only because of the rarity of legacy Exeggcutor. Now that confusion is no longer legacy and psychic has been improved, Exeggcutor is probably the better pick.

Did you have any other pokemon you thought might be worth boosting? Let me know in the comments below!

Edit: Added Farfetch'd, Azu, Delcatty as per comments below.