Portal 2 is the true definition of a sequel, while Portal 1 was more of a technical demo. Portal 2 is a full-fledged game. A superb level design that has you solving puzzles using only one blue and one orange portal with clues subtle enough to not feel like you're being hand holded the entire game. A story that surprises how good it is despite only using 3 characters (of which only 2 actually speak) and an ending that to this day I still cannot forget.