hm... i always had a hard time choosing a favorite type. it would be a mix of ghost, psychic, and dark. having ice, dragon, or fairy type would be cool too though. i'm sad that i'm not as familiar with the specific pokemon as i used to be but... soon.
Ground type. My favorite pokemon are mostly ground types anyhow. Not to mention that the ground type has some pretty nifty type combos like Flygon (dragon/ground).
I think I'd train Poision type Pokemon. It's been awhile since I've played Pokemon, I usually had a mix of psychic, Poision, and dragon types in my party.
Water...I love the water, and it'd give me an excuse to spend a TON of time in it training them. Plus, a lot of my favorite Pokemon over the years have been at least part water types, so more power to me!
Honestly, for my gym leader role on the forums I purposely picked a type I didn't care for (bug) so that I'd be forced to learn the ins and outs of the type. It's been eye-opening, to say the least.
I would choose actually
Ice type:because they are a powerful type they fight tough and they are really cold which is an advantage on other types and they also have cool looking pokemon.
Dragon types: I would call my self a dragon leader at times cause I think dragon types are powerful and cool I could train them to the highest strength but also keep them clam like iris in pokemon black and white.
Grass types: I will rule the world with grass types they will raise again.lol but they are my favorite types.
I would probably be an electric user. I like fast pokemon like Galvantula, or strong pokemon like Electivire or Eelektross. I could be a bug user because I have a killer gen. 5 Durant, Shedinja, Galvantula, and Volcorona.
Maybe I'd be a mixed type. I'd want a Nidoking on my team, and a Ditto, and a Megansium, Blastoise, and Charizard. Don't know about the last one though.