Don't get me started on the third gen, I fear if I start listing off my favourites it won't be long before I've listed nearly every single Hoenn pokemon.
If I'm being really strict though...:
Favourites: Numel/Camerupt, Roselia, Seviper, Mawile, Torchic/Blaziken, Salamence, Flygon, Swablu/Altaria, Gulpin/Swalot, Kyogre, Tropius, Lunatone, Corphish, Ralts/Kirlia/Gardevoir, Snorunt, Clamperl.....etc etc
(See what I mean, there's just so much quality....I could go on and on) Numel and Roselia are my absolute favourites of the bunch though.
I started appreciating Salamence, Flygon and Altaria in later generations though. It's funny how that happens.
I can't say I dislike any of them, the most I could say is I feel indifferent towards a couple like Luvdisc or Electrike/Manectric. Actually that's not completely true, the internet has ruined Mudkip and it's family for me....I just cringe whenever I look at it now.