I have Dark Heresy, and am interested enough to decide to start prioritizing the order I'll get the other games in. To that end, of the other three, which do you think is the most compelling? They each have their draw (Rogue Trader has space ships, Deathwatch has Space Marines, Black Crusade has a wickedly original premise).

What do you all think?

The big question is, what do you want to play? Because in playing styles the three games you mention varies alot between each other with some similarities (I think) between Rogue Trader and Black Crusade while Deathwatch is an entirely different species of RPG.

I haven't played Rogue Trader but understood it to very much be a high-level sci-fi adventure where you lead a ship in a dazzling adventure in search for profit and glory into the unknown reaches of space

Deathwatch is very much a military RPG and thus players who love thinking tactically and working as a team to take down monsters and enemies in various scenarios will love this - but more role-playing oriented people might be a bit put off by the focus on action and tactical thinking

Black Crusade is of course rather much an adventure game where you play the bad guys and instead of putting a halt to their diabolic plans you are actually planning them out and carrying them through. The big drawback is probably that playing bad guys who are probably going to come to blows with each other isn't appealing to all groups and players - even if I love it and some of my best RPG experiences has come from playing games that were ok with inter-party conflicts and violence.

Rogue Trader is a huge sandbox, the players get a ship, powerful characters and an aim: wealth and power. The how is left for them to decide. If you always liked your characters with a freewheeling flair and if you want to have a hand in the direction the game is going, then RT is for you. Contrary to DH you don't play wimps, you play some of the best mortal characters in existence.

Deathwatch on the other hand is a tight game, you play a team of space marines and your GM makes the missions. There is less freedom of choice than in RT, but DW has enough to make almost anyone happy. Playing an SM is something that trumps playing many other characters. There is a drawback in the fact that the game offers rules for combat but the role-playing must come from inside. You can have great RP moments, but all players around the table must want that and work towards bringing RP to the table. Otherwise it's a tactical simulation.

Black Crusade is another sandbox, this time with a twist. You can have a team of chaos space marines and mortals, and one will not overshadow the other. You also have many freedoms when it comes to playing, tough as with RT there's a distant light at the end of the tunnel in form of daemonhood. Chaos is not everyone's cup of blood, but I've found BC a nice diversion from DH.

In my opinion RT has a natural synergy with DH. A lot of stuff can be ported from one to the other. It harder with the other systems.