Just some frivolous thoughts, as I was playing through AC:2 and found its feather collectible to be so satisfying especially the balloon popping sound, having played through most of the legacy AC titles I find BlackFlag (chasing sea shanties are the best), AC2/Brotherhood, AC3 to have the best collectibles in general which actually serve a purpose and not too abundant.

Rouge/unity easily has the worst since you have collectibles littered all over the place and I still dont have any incentives to be collecting them, Syndicate is somewhere in the lower end, ithas some pretty cool illustrations, flower remarks and beer tasting notes but the large map plus the amount of them put me off. Revealation is somewhere in the middle.

Haven't played the new RPG-inspired AC titles so I don't know about its collectibles system.