Rogue Trader! Or other 40k FFG games.
But the main preference goes to Rogue Trader, for it satisfies my fetish for ship-building needs for complex Imperium politics.
I'll have to admit that I'd like to try it as a solo game - I kind of want to see just how crazy a Lord Captain can go in the quest for Eldar booty riches and ancient artifacts without a table of people around to tell him 'No, wait a moment - that's STUPID', but I'd of course be open to group play.
If the GM we find (of course we'll find a GM) is truly a saint (and probably unemployed, from the time it'd require), it might even be interesting to set up a game with either multiple families or multiple branches of the same - each with their own ship and doing their own thing, with the occasional uneasy alliance to loot a whole world before the Everpresent Ominous Timer of Certain Doom reaches 00:00.
I do know a bit of 40k lore, but I'm not an expert by any stretch of the imagination - basically, you can expect me to know what can be found on a wiki, but not to have read any book. I'm also not particularly a purist - especially when it comes to Rogue Trader, so feel free to surprise me with your interpretation of the 41th Millennium.
I'm also open to stealing mechanical bits from the later games - I know several things made significantly more sense in, say, Black Crusade than they do in Rogue Trader (I haven't played in a while, but I remember Black Crusade at least gave you a reason for single shot attacks, and unnatural attributes were less insane). Since I'd be on the side of the GM screen with less headaches, I'll leave up to the GM to decide what to steal.
The other games are lower in preference (Roughly Rogue Trader > Black Crusade (played with some class) > Dark Heresy > Only War = Deathwatch = Black Crusade (played with mostly dakka)). I'd be at least intrigued by a Dark Heresy Ascension (Or similar concept in DH2E) game with a peek at 'big picture' issues (I played in one here many moons ago, and it was interesting until it crashed and burned).
A note for anyone who read this far, my ship-building fetish would probably also be satisfied by a Traveller game, but I only played that once, several years ago - so I'd be super rusty.