I have too many favourites! You can see this is my favourite gen for pokemon.
My favourites include: Numel, Camerupt, Roselia, Tropius, Seviper, Lunatone, Solrock, Salamence, Nosepass, Torchic, Blaziken, Treecko, Sceptile, Castform, Kecleon, Surskit, Mawile, Trapinch, Flygon, Swablu, Altaria, Baltoy, Claydol, Feebas, Milotic, Duskull, Absol, Chimecho, Snorunt, Spheal, Gulpin, Swalot
There aren't many from this gen I dislike but here's a few that I have never warmed to:
Electrike, Manectric, Vigoroth, Meditite, Medicham, Plusle, Minun, Aron, Lairon, Aggron, Metang, Metagross