Definitely stealth rock, but bullet punch has definitely helped my team out quite a bit!
Ah of course, who could forget the infamous Scizor+BP...
Speaking of which, BP should be up here...

Definitely stealth rock, but bullet punch has definitely helped my team out quite a bit!

no doubt DD. without it we wouldn't have witnessed the biggest choke in mons history: justinawe vs Gouki
I can't believe no one's said this yet--Hidden Power! Suddenly, any Pokemon can learn a move of any type, thus giving it a strong counter to a single Pokemon that might otherwise be its downfall.
HP Grass Zapdos laughs at Swampert!
HP Fighting Rotom pwns Dark-type counters!
Hidden Power allows so many Pokemon to have perfect coverage, and EVERYONE can learn it! No question in my mind--it is the best attack in Gen III, IV, and probably even V (we'll see).
But it only applies to specially-inclined pokemon - Machamp's HP anything won't dent apart from a four times weakness. Yeah, it's a good move, but there's no way a 70 BP move should be crowned the best of Gen IV.
U-Turn is on the list, which is also 70BP, and MOST moves can only be used effectively by a select few. Hidden Power is used effectively by far more pokes than any other move and works out to be the linchpin to many a moveset.
Stealth Rock is #1, obviously. No other move has ever centralized the metagame as much as those floting rocks have. Games are won and lost based on this move.
Sure, Spore is amazing, not saying that it isn't, but it takes one Pokémon out of commission temporarily, while Stealth Rock affects all of your opponent's Pokémon for the rest of the match (unless they pack a spinner), so it gets my vote.
Actually, if I could vote for a second place, I would have went with U-turn, simply because I LOVE the scouting and tempo advantage it gives. I almost never run a team without at least one U-turner. Although I'm probably valuing it higher than I should simply due to personal preference. =P