If it was anything in that universe, I'd go with a techpriest on Mars. Being safely distant from fighting and spending all day trying to make machines work is a life I can relate to. In the grim darkness of the 41st millennium, there is only poorly documented C++.
Now, if you're asking for an actual front-line Codex entry, I'd go with a farseer. A nice long lifespan, the ability to predict the future, and having a leading position with an excuse to not be at the front of the charge are some nice occupational perks. The whole having a god specifically aiming to eat your soul thing would kind of suck, but hey, you can't win 'em all right?