Definitely Plants vs Zombies.
I'll come out and say it: I HATE TOWER DEFENSE! And yet, PvZ is so good that I don't even think of it as such. The replay value is excellent, as there are loads of mini-games, survival modes, and puzzles to play long after you beat the Adventure Mode (and the New Game+). The Zen Garden idea from Insaniquarium makes an appearance too, which keeps the game more relaxed for those who just want to grow plants.