Probably the Dreadnought. It's something we don't have now and I suppose this exotic-ness adds to the appeal. Its also very grimdark (piloted by a half dead dude, c'mon) which makes it fit with the
40k style perfectly.
Aside from that, the fact that it is a multiple ton killing machine that can be outfitted with all kinds of closed combat and ranged goodies doesn't hurt
My second might be a Basilisk. I've just always liked artillery.