There is an argument to be made that we shouldn't get so hung up about graphics, but in a game so based around exploring a unique environment, graphics are truly part of the experience. Primarily, it simply makes the game much, much prettier. With that in mind, I'm very much enjoying replaying SHOC with the Stalker Complete 2009 modpack.
While the Oblivion Lost mod and its million features is the most renowned fan-tweak, there's much to be said for retaining the game's original structure rather than necessarily embracing such sweeping changes. Jim's already done an extensive round-up of the more interesting mods for Stalker: Clear Sky, but me, I'm happy sticking to the original Stalker: Shadow of Chernobyl.