Doom 3. The first time i got this game i played it non stop and it had multiple levels in the campaign, and the story line was just amazing. Yeah i know its a really old game. Not a lot graphics like some of the new games, but it was a "GREAT" game. If I want a good game to play or just a scare i would put this game in my old Xbox or 360 and play it all night. You just can't beat a classic. Like Doom, Doom 2, and my favorite Doom 3
That is a personal choice but if really want to play a good "story line" i suggest Black Ops. It seems that they had focused more on the more on the campaign then they did the multiplayer it has less perks and weapons then MW2 but some different stuff as well. Also if may like getting Black Ops if you want to play the Nazi Zombies too. Now if you want to spend hours on multiplayer i would suggest Modern Warfare 2. It has multiple weapons, perks, add-ons and many different ways to play like teamdeath match and capture the flag. So its your choice now either Black Ops or Modern Warfare 2 or you could get them both.