I didn't like TF2. Thought it was a spam fest, and after a week it got old.
The Orange Box is worth getting though. You get HL2 and Episodes 1 and 2 (there's a third episode in the works). This is a great single player game. Yes its old, but you don't really notice it. Graphics and the engine are still great.
Portal is a good puzzle game, after completing it there is a challenge mode to change things up, extending the gameplay.
Now, once you are done with that, whats next? MODS! Owning a game that uses the source engine gives you access to the SourceSDK in Steam. With this, you can play quality mods like: Insurgency, Eternal Silence, GoldenEye Source, PerfectDark Source, D.I.P.R.I.P., and the list goes on with more on the way! The ones listed are online multiplayer, but there are single player mods available as well, like Black Mesa Source (this one is still in development, but I can't wait!).