Renting wise, I'd say Devil May Cry 4...because there is not much to do with Dante or Vergil (the stars of all the other DMC games in the franchise) you won't be sitting there going "eh?" the whole time.
It's got a pretty good storyline, lots of action and a decent level of replayability too...
If you were to buy a game...well, I'm torn between ME and LO personally. For the sheer complexity and depth of the storyline and the beautiful graphics I'd say LO, but you need plenty of hours to play it. ME is excellent but whilst there is quite a bit of replay you can do, the main storyline could get a little tedious, for want of a better and possibly less inflammatory word, after the first two/three playthroughs and whilst you have plenty of choices to make, they aren't without limits, obviously. But it's an excellent choice all the same