it really depends on you. im a well rounded gamer so here's my list.
mass effect and assassins creed series for long deep gameplay.
skyrim, gta, fallout and whitcher series for open world gameplay with alot of side questing.
resident evil and dead space for horror.
dirt 2 and 3 for racing game that you'll enjoy and keep putting in down the road. this personally applies to EA's skate as well. i still play it now and again.
halo series. personally pick them over new fps games like black ops n such cuz i feel i get more out of them.
portal series for its puzzle solving.
beautiful katimari for all age clean fun.
for arcade type fighting games i pretty much get the same level of satisfaction out of all of em but i suppose mk vs dc or soul caliber are the tops. lot of good inde games as well as classic and xbox originals on the live market place to. some Dont give you achievements tho.