Now that I think about it Return to Ravnica is actually a bit underwhelming in retrospect, and I can tell you from personal experience that you should never, ever buy a Dragon's Maze box. No, not even then. So that pretty much leaves Khans by process of elimination, which is fine seeing as even if you don't like it as a set it's chock full of individual bits of awesome (SIIIIIIIEEEEEGGE RHINOOOOO!!!!). Not to mention it's almost certainly the one that will net you the most trade/resale value due to being the only one of the three currently in Standard.