If you can only buy one expansion, Opposing Force is the clear choice.

Fortunately, there is absolutely no need to risk your PC's security with untrusted torrent networks. Valve actively maintains the GoldSrc engine ecosystem on modern digital storefronts.

While Half-Life placed you in the shoes of Gordon Freeman, a scientist struggling to survive, Opposing Force flips the script. You play as Adrian Shephard, a Marine dispatched to Black Mesa to eliminate the alien threat—and silence the scientists. 1. The Perspective Shift