This is a fun thread, really cool idea. Very difficult to think though.
Grass- Probably the Sceptile family. I've always loved the reptilian look and his mega form.
Water- Empoleon, hands down. Beastly SP.ATTK and the half-steel typing can make him very hard to kill.
Fire- Magmortar. I've always loved magmar, but ever since they gave him an evolution, I've just fanboyed so much on him.
Electric- Magnezone. Tied for my fave of gen 4 with Magmortar. Simply awesome.
Bug- Kricketune. I would've given this spot to Vikavolt, but Kricketune has saved my butt more times than I care to count in B/W.
Flying- Talonflame. Just... Nuff said.
Normal- Slaking. Hes too fun to use sometimes with a Hyper Beam/Giga Impact strat.
Fighting- Machamp. He's got power like Conkeldurr, but much much faster. I love him because he's good at what he does.
Rock- Gigalith. Sturdy, only having 2x weaknesses, and being a tank are all very good things for a rock type.
Ground- Sandslash. Ive always had a soft spot for the sandshrew line because he's so frickin cute.
Steel- Used to be Steelix, now it's Aggron. What a champ.
Poison- Crobat. So many good ones to choose from though.
Psychic- Alakazam. Glass, Frickin, Cannon.
Ghost- Chandelure. Such a cool design, and a great 'Mon.
Dark- Umbreon. My fave of gen 2. Still my fave now.
Ice- Walrein. Because I had no idea how good he'd be til I actually used him. The grass weakness is strange though.
Dragon- Rayquaza. I dont usually count legendaries, but this pokemon is a BEAST. Just got done beating OR/AS again and the delta episode. <3
Fairy- Florges. Surprisingly good, kind of a tank. I like it a lot. Didn't think I would at first on account of it being a flower.