Most excited for right now is probably Arceus, though that's likely because she's in the upcoming update, but I do have some pretty good plans for her, having the literal GOD of the world in the harem should lead to some fun scenarios too!
For future mons, I've got something planned for Gen 6 to make up for the lack of Sub-Legends in those games, but I won't go into detail about that yet.
I'd say I'm not looking forward to Zeraora or Zarude, their designs aren't particularly interesting considering they're mythicals, there's not a whole lot of lore to work with either.
My least favorite Legends out of all of them are the Loyal Three, but they have good lore to work with and their designs can be improved easy enough, so I'm actually kind of looking forward to them!