Let's build a character who's optimized for slaying dragons! Some observations about dragon slaying in D&D:

1. Legendary resistance and high saving throw bonuses make it difficult to slay dragons using saving throw-based mechanics.
2. An AC of under 20 for many kinds of dragons makes them much more susceptible to attack rolls.
3. They all have a Frightful Presence which requires players to make a WIS save or get debuffed.
4. They all have a breath attack of some element or another which requires players to make a DEX save for half damage.
5. They can all fly and can often burrow or swim.

Offensively, a dragon slayer should probably not rely on saving throws. Without relying on support from others or magical items, relying on melee is probably a fools errand as well. Ranged attack rolls seem ideal here.

Defensively, a dragon slayer wants to have boons to WIS saves to avoid fear conditions and DEX saves to avoid breath attack high damage rolls. They also want a high AC. Getting access to things like Rogue Evasion, Absorb Elements, or some form of courage mechanic is all going to help a lot.

One sample build foundation that uses a class I really like but I don't see it come up often is a kobold inquisitive rogue. Using MMoM rules, Kobolds can come equipped with defiance, which gives advantage on WIS saves to avoid or end the fear condition. The inquisitive rogue subclass offers a path to Rogue Evasion without relying on hiding to get sneak attack; this is important because dragons all have really high perception. I also really like the flavor here of a kobold archer who slays the dragons who enslaved his people. At my tables we get a first level feat at character creation, and Sharpshooter stands out to me mainly because of the lack of disadvantage at long range, which will help with dragons flying around.

A fun, pure PHB spell caster build to consider is a tiefling fiend warlock eldritch blaster. Agonizing Blast checks all the offensive boxes required (especially since it is force damage so you won't get countered by some dragon types like you would if you relied on, say, firebolt), and the 10th level feature of the Fiend subclass lets you suit up for a dragon fight with the appropriate damage resistance turned on already. Add to this the WIS save proficiency and a lot of the defensive bases are covered as well.

Thoughts from the crowd on these proposed build ideas or other ones?