Well Team fortress 2 is an unlimited game, Portal is one that you can finish within hours (but still, they'll be some fine hours) and Half life 2 is a very long game if you have both the episodes as well. Depends what you feel like. If you feel like shooting zombies and combines go for HL2, if you feel like blowing up/burning/snipering/generally shooting others go for TF2 and if you want portals go with Portal.
The difficulty of the puzzles are inversely proportional to GLaDOS's sanity(or whatever sanity an AI can have).
One playthrough is about all the challenge there is. After that are just the Advanced Chambers, which are so Nintendo Hard that it's not even funny.