I'm gonna be a rebel and say Pearl (Though, if I had gotten Platinum, too, I would have probably preferred that.) Gen IV is where Nintendo really got their shit together, I think. Like B/W, the major focus is on the new Pokes, but unlike FR/LG and B/W, the game doesn't limit you to a ridiculously small, unvaried, mediocre selection for your main storyline team. Sure, B/W has re-usable TMs and the new Battle set-ups, but D/P absolutely revolutionized the battle scheme with the physical/special split, HG/SS has the best controls (with the Touch Screen serving as a near-perfect substitute for the right hand buttons), and the movement to DS graphics and music blew me away.
Oh, and the items... I really miss the 2-Speed Bike, Underground Kit, Poketch, EV held items, and especially the VS. Seeker. B/W has an odd obsession with multiplayer (C-Gear, Entralink, Dream World, IR, Local Wireless) that doesn't fit a 1-player RPG, and even significantly impedes what the game could have been (namely, because the C-Gear renders the Touch Screen, so useful in HG/SS, totally POINTLESS.) Also, the main game in B/W, even including the post-Ghetsis stuff, is far too short.
So, by Generation, I'd say IV > II > III > V > I.