Portal 2

Valve took the surprise hit that shipped as a bonus in The Orange Box and built it out into a full game without diluting what made it work. You still solve rooms with a gun that puts one end of a hole here and the other end there, and momentum still carries through, but the 2011 sequel adds gels that make surfaces bouncy or frictionless, light bridges, tractor funnels and excursion funnels — each introduced in a handful of teaching rooms before being combined. The puzzle design is unusually generous: solutions are rarely obscure, and the satisfaction comes from seeing the shape of a room resolve rather than from brute-forcing it. The writing carries the rest. GLaDOS returns with a grudge, Wheatley is one of the great comic performances in games, and the pre-recorded ramblings of Aperture founder Cave Johnson fill in a corporate history that gets steadily more alarming the further underground you go. A separate two-player campaign, built for two portal guns rather than retrofitted, is still the best co-op puzzle design around.