PCCS aka Phoenix Command, is brutal both in combat results and the amount of work required by the GM to get to those results.
Personally I've only seen the (slightly) watered down Aliens (as in the movie) book but that's deadly enough to give even munchkins nightmares. Throw in the power armour of Living Steel and watch out for the rivers of blood and pulverised flesh.
http://en.wikipedia.org/wiki/Phoenix_Command
Leading Edge is no longer trading (folded in the late 80's I think) and the books are
extremely hard to come by. With most other game systems you can probably download a torrent, but with PCCS those few who have the game are absolutely petrified of copyright laws. Used hardcopies, on the rare occasions when they are available, can sell for hundreds of dollars per book.