I voted for Pokémon Yellow. I haven't owned Pearl or Diamond so I thought it wouldn't be fair to vote for Platinum. I've never played Emerald. I've forgotten what Crystal's like as it seemed similar to Gold. And Black/White 2 aren't much better than Black/White.
So Pokémon Yellow was my favourite at the time but I like Pokémon Black/White 2 most.
My top 5 favourite Pokemon (subject to change) - #5 Toxicroak #4 Miltank #3 Hariyama #2 Blazenkin #1 Pikachu/Raichu
Platinum gets my vote. In my opinion, it's the third version that most improved on the originals. I'll still play Red/Blue, Gold/Silver, Ruby/Sapphire (I actually prefer these to Emerald), and Black/White. I haven't even wanted to touch Diamond/Pearl since Platinum came out.