READ THIS POST, IT ACTUALLY CONTAINS HELPFUL ADVICE AND ONLY A LITTLE BIT OF FANBOYISM!
LOL I GUESS YOU ALREADY DECIDED WHAT TO GET BUT I WILL KEEP THIS HERE FOR INFORMATIVE PURPOSESWould you rather build physics contraptions and forts, or would you rather shoot people?
If you want a sandbox game (like Blockland is) get Garry's Mod.
If you want an FPS get TF2. I've heard CS:S sucks, and I think TF2 is pretty fun myself. But I believe CS:S does have has a decently sized fanbase, so some people do like it.
People who say things like "GMod is best" and "GMod is better than both of the other ones" are comparing apples to oranges. GMod is a sandbox game and CS:S and TF2 are multiplayer FPSs. Really the choice depends on what you like better.
BUT IT SHOULD BE NOTED THAT YOU NEED A SOURCE GAME TO PLAY GMOD. SO IF YOU DON'T OWN ONE OF
THESE GAMES YOU SHOULD PROBABLY GET THE GMOD / TF2 or GMOD / C:SS BUNDLE. If you want a game to play get the GMod /TF2 bundle. If you just want more props and less broken textures, get the GMod /CS:S bundle.