Small: Defilak or Idris. The Mahritoran had great builds, imo the best of all matoran we got, what sets Defilak and Idris apart from Dekar and Sarda are the masks. Goood those masks...
Toa: I would say I stand with the somewhat unpopular opinion of saying that the Inika probably are the best Toa sets. That is a discussion on its own, but I would say that my favourite of them is either Matoro or Nuparu. Least favourite of them is Hahli, and not because of the mask, its because of all those melded blue/white pieces. Ew. Honorable mentions are Vakama Metru, Lesovikk, Lhikan and Lewa Phantoka.
Glatorian: For looks alone: Gelu and Stronius. For inventive parts usage: Skrall and Vorox, if it wasn't for those blue pins in the arms and legs.
Villain: Krika, Antroz, Takadox, Pridak, all the Piraka and all the Rahkshi. Let's be honest, the villain sets were probably the most consistently good we got.
Titan: Brutaka. No contest.
Vehicle: Axalara. Never owned any of the vehicle sets, though I always thought Axalara was clearly the best looking.
probably unpopular opinion time: imo there were way too many vehicles in the later years of G1. Sure most of them had great functions, but they also generally had very meh characters. Also,
I think it was very limiting in the storytelling, because we could have gotten much more from 2009 if some of the vehicles were replaced by a titan or something like that. Imagine if we could get maybe a Vorox cheif or alpha as maybe a 29$ set, that would bring a whole lot more depth to the story instead of "Here's a guy. He has a motorcycle.
Yay." Rant over.
Small: Uxar, probably.
Toa: Kopaka Uniter
Villain: LoSS Not a titan. Fight me.
Titan: Umarak the Hunter