Definitely the bugs -> Tyranids (and Zerg, among others)
I guess ya, the homo sapien vs adeptus astartes is no contest, but I was thinking the Mobile Infantry with rocket boots and tactical nukes on their arms... that sure beats running on the ground with a bolter and a chainsword :-P
True, but then again, in 40K, we have terminators, melta bombs, standard-issue RPG assault rifles, assault marines with jet packs, dreadnoughts with heavy artillery...
Arm-mounted tactical nukes though. For real :-P