I love this question... There are a few that I'd absolutely love the chance to play - even in whatever unfinished state they were in. There are a good number of games that we either know were pretty far along in development or even close to being finished. It'd be hard to pick one...
There are some obvious (and good) picks, like Star Wars 1313, Pray 2, Half-Life 2 Episode 3, FF Versus XIII, the seemingly dozens of cancelled Legacy of Kain games over the years... but there was one cancelled game that I feel like I'm the only one who actually cared about playing: Highlander.
Was there any chance for this game to be much more than a swimming-in-sevens? Probably not. But ugh, I really thought Highlander was such an untapped potential for video games, and I just really wanted to play through this surely mediocre time-spanning action game!