Portal 2, followed by Rayman Origins, and then Dark Souls. I prefer the original Portal to the sequel just slightly, because GLaDOS is much better in the first game, and there was a slightly more sinister undertone when you started to discover the back rooms and piece together the lore. Plus of course it was the original and therefore the innovator. Portal 2 had the benefit of being a full length game, adding an extra dimension to the puzzles, and introducing some other strong and amusing characters, so it was an excellent sequel.