Yellow version!

Pokemon Mystery Dungeon forever!
If they don't count, then Emerald.

Black. Ain't played BW2 yet.

I've been playing since Red and Blue, but I found the improvements in the general system absolutely exhilarating.

A tie between pokemon yellow or pearl

I loved Diamond the most. ♥

Diamond. I like the fourth generation the best. Yeah, I said it. Get over it. FOURTH GENERATION FTW

Ruby and Sapphire FTW.

Emerald, Leaf Green being a close second.

Last one I played was Platinum, I have serious catching up to do in the series.

Nothing will ever beat 1st and 2nd Gen. R/B/Y, G/S/C.

Everything else is 2nd to those

I liked Gold/Silver and Heart Gold/Soul Silver, but I really want to try Black and White!

Emerald & White.

Soul Silver.

Oh come on, nobody is going to say Pokemon Black? I thought it was amazing for one in the main series

As far as spin-offs, Pokemon Mystery Dungeon: Explorers of Sky. Which one of those two? I thought they were both great games in their own way so I can't really decide the favorite out of those two.

Soul Silver, I loved how the pokemon sprite could follow your trainer around, I wish they had kept that for Black and White as well

Emerald and Heart Gold

HeartGold
All the good stuff of the newer games, but with the awesome old pokemon!

Emerald.
Followed by SoulSilver because I really think they should make the lead Pokemon walking behind you a standard thing like the animated sprites and stuff.

Coliseum. Or Pearl. Emerald would've won, but I love the physical/special split too much