More of an extremely enhanced remake of one of the greatest multiplayer FPSes of all time, CS:GO's got a ton of replay value, extensive Steam Trading/Workshop support and much more. Think Counter-Strike: Source, but with a substantial e-gaming support, user trading and customization layer added in for funsies. I cannot recommend it enough if you've ever played CS before, or if you're looking for an excellent example of simple yet challenging FPS gameplay in an easy-to-run, well-supported and popular package.
Originally written October 8, 2014 on Steam