StarCraft 2 because i haven't played all the other games
I'm assuming your mean StarCraft: Brood War, and I agree. No game has more replayability and better online play! Hopefully Starcraft 2 doesn't dissapoint.
I would have to say all Blizzard games are relatively good. Granted, I haven't played Warcraft 1&2, or Diablo 1, but I feel the versions I played were great (Warcraft 3, Diablo 2, Starcraft and Brood War, and WoW).
I can't imagine the Blizzard Company without Diablo II: Lord of Destruction! It was the best RPG I ever played... It had a great story, good and balanced spells and 7 great classes to play! Warcraft III was also a great Strategy Game but it gets bored after playing the same strategy too often... Can't wait for Diablo III.