SHC: Luffy or Usopp. One is the protagonist and the other has had the best character development in the series. I'm pretty confident that Oda not only favours these two but also enjoys their interactions the most because they're the juiciest among the crew because of the parallels and contrasts between them.
Marines: Garp I guess.
Worst Gen: excluding Luffy and Zoro for obvious reasons, I'd go for Blackbeard. Has been there the longest and feels like a character very funny and interesting to write, like other Oda's favourites like Gaimon or Buggy.
Whole Pirate Era: I'd say Doflamingo and the Donquixote Family overall. The amount of detail, good designs, subtlety, synergies... among that group of characters is amazing and they feel like the most authentic, passionate work by Oda excluding the conflict between Luffy and Usopp.