I think my favorite may actually be Johto. Feraligatr is just plain awesome, Typhlosion is not too far behind and I always like Chikorita and its evolutions' designs. "Viability" is obviously an entirely different question (even within the Johto games, poor Chikorita) but I don't care about that side of Pokemon nearly as much as I did before.

My least favorite set is Kalos'. I like Braixen due to its character in Pokken and...that's literally it tbh. Well, Froakie's 2nd stage is okay as well I guess (altho I can't think of its name right now) and I just plain don't like Greninja's regular color scheme. It looks weird to me, black works a lot better imo but how often are you actually playing through one of the games' single player portions with a shiny.

Runner-up to Kalos would probably be Alola tbh. Hate Decidueye and the other two are just..kinda average I guess. They look plain and overdesigned at the same time to me, if that makes any sense to you guys :P

I actually really like Unova and Sinnoh as well. Don't really care for Samurott or the monkey (names, how do they work..) but I love all of the remaining 4's designs.