It is actually really hard for me to pick the ONE that I like MOST.
HG/SS was big. I mean, the endgame is another game. I don't care about my Pokemon following me. I liked some of the uses on the touchscreen. Generations II & III were never that great for me though (I don't like hardly any of the starters).
B/W is big (in a different way). Over 150 NEW Pokemon....more than any other generation. Period. The starters were pretty interesting (to me) and this is the first time I have ever used anything other than fire. (Side note: Game Freaks come on. I mean, three generations in a row with Fire/Fighting? Really!?!? Shame on you!!!!) I haven't gotten to the endgame myself, but I do understand it's pretty lackluster. While there were some unique features in the C Gear, really did not care for general touchscreen uses/implementation. Graphics are pretty good.
Now, Platinum on the other hand.... Loved the starters (okay, except for Turtwig). GREAT use of the touchscreen (though I did like HG/SS putting key item buttons on it). AWESOME endgame. All around generally cool Pokemon. So yeah, Platinum FTW, hands down.