I voted for Spore.
Stealth Rock easily takes second, because it while it may not be the best move in the game, it has certainly been the most metagame defining. Every Pokemon is damaged by Stealth Rock (except Clefable) and it only takes one turn to set up, which automatically makes a ton of moves (Eruption/Water Spout), items (Focus Sash), strategies (Substituete+Reversal/Flail), and Pokemon themselves unviable. This one move has the power to decide the fate of Pokemon like Yanmega, Moltres, and Charizard, who would all be a tier higher without it. It defines the lead metagame itself, you either set it up, prevent it from being set up, or punish the opponent for trying to set it up. Nearly every team uses it, damage calcs take it into account by default, guarentees 2HKOs and 1HKOs, ect.
However, consider Spore. It's so overpowered that it has to have a clause devoted to nerfing it's effects. Without Sleep Clause, the game would be completely different, despite only three Pokemon learning the move. Even with the clause, Breloom secured itself a spot as a top-tier OU because of it.
Compare it to Encore, another amazing move that made the options list. Encore's practical effect is giving you a free turn by forcing switches. Spore does the same thing, except that Pokemon is now incapacitated for the rest of the match. With Encore, if you're not fast enough you may end up encoring an attack. Or the opponent may predict it and switch. In both cases you just wasted a turn. With Spore, neither of these scenarios matter.
Pick a random NU Pokemon, and give it Stealth Rock. It's not much better than before, is it? But pick one and give it Spore, and it just got a whole lot better.
Look at Smeargle, the master of versatility who can learn every move in the game. What's the one move you can except from it? Spore.
My list would probably be:
1. Spore
2. Stealth Rock
3. Substitute
4. Draco Meteor
5. U-Turn or Encore
Draco Meteor should definitely have been an option.