Before I had my hands on Platinum, having played only LG, Emerald and SS before, I'd describe myself as a genwunner. Sinnoh changed a lot for me, having many of my favorites in it. Also it gave me a greater perspective for the new generations.
I've always considered my favorite typing to be Grass and/or Bug, but weird enough, after Gen 4, most of my likes were from Dark-typing. This is going to be a long list here, here I go.
First off, Weavile, my second favorite pokemon of all time, had a big impact on my viewpoint back then. I love everything about Weavile, and also Lucario.
After Blaziken, my favorite fire-starter is Infernape. I really want a Gen4 remake to see a Monkey-King mega evolution for Infernape. Also I loved Chimchar in the anime.
Staraptor is one of my favorites when it comes to early-region birds. It's pretty strong and looks cool, just like Luxray, which, I think, needs the Dark typing.
Although I've never been a huge fan of water Pokemon, Buizel line is definitely one of my favorites. It's design and it's sprites are so cool, I loved them.
Leafeon, my favorite eeveelution was introduced in Gen4 as well.
I'm in love with two Poison-typed pokemon from gen4; Drapion and Toxicroak. I've used Drapion as well but never had chance to use Toxicroak. I love their designs and typings.
Also, Gliscor and Froslass deserve a honorable mention here.
When it comes to legendaries, I don't really like anything besides Darkrai. I've prefered Giratina over D/P counterparts all the time, so there's that.
I had trouble choosing my least favorites ones, but there are two steel types that I never liked their design, being Bastiodon and Bronzong. I like everything else about Gen 4, I think it was one of the most successful generations in my humble opinion.