So it's been like 5 minutes since we last had one of these topics, figured it was time enough for another one. To avoid rabid fanboys clamoring over Uncharted and Gears, we're making this multiplat only. Which unfortunately does exclude The Witcher and Souls series. Sorry Sean.
So... let's do this. Of course you can argue any IP, granted that it is a multiplat one, but I figured these are the most popular in order to encompass the majority of the posters' opinions.
My personal vote... Mass Effect :D