Diablo.
Should I buy and play Diablo II for the first time, or buy and finish Diablo 3?
Should I buy and play Diablo II for the first time, or buy and finish Diablo 3?
It depends on whether or not you can enjoy an older game. D2 will give you a lot of the background of D3 (though not all, as the rest is in D1 of course), but D3 can be enjoyed without knowing the full story if only for the gameplay.
For a casual gamer I'd just tell you to play D3, but if you enjoy experiencing the full story I suggest you play D2 first (and D1 if you're really into it). D2 and its expansion LoD should be pretty cheap, if not free if you get games the shady way.