I liked Half Life 2 as a first game because I got to learn the basic keys/commands without having to deal with all the menus and layers of fluff in other games. The game I liked the most so far was Stalker Shadow of Chernobyl with Stalker Complete Mod. Its a simple game mostly but very inventive. Just the sounds of the gasps of the dying characters is pretty wild. Things like anomalies, atrifacts, ghosts that can be shot, wild mutant animals etc are more imaginative than the standard stuff. Oh Left for Dead2 is also pretty simple but still fun. Valve follows the KISS principle and I like it.