If you mean an online RPG,I would recommend World Of Warcraft,that's the best MMORPG ever! But if you want to play a singleplayer RPG,you can try Fallout 3,or The Elder Scrolls of Oblivion.
one of the best rpgs ever made was planescape torment. i know it is pretty old but it was really different than most rpgs. i liked the fact, that the hero could not die(well it was your task to kill him) and it put less focus on fighting but more on questsolving and interessting dialoges. for all of you guys who enjoy some old school rpg that is more than just hack and slash this is definitly a game to play
Stalker: Shadow of Chernobyl. Not common in Gamestop, but if you find it, buy it. Bear in mind, it's only because the post-apocolyptic theme appeals to me.