1. Garry's Mod
Pros: Great physics. Addons are easy to make. Lots of other fun addons. Make ALOT of neat contraptions. Makes good movies/videos, if you know what you're doing. Good thinking NPCs. Pretty nice graphics. Good achievement system.
Cons: Community is bad, lots of trolls, and crybabies. Creator is a mad bull. Good amount of addons have LUA errors and some don't even work.
2. Blockland -
Pros: Ok physics, Lots of bricks, No lag. Can easily reach 300+ FPS. Great eventing system. Costs $20 and never have to pay again. Addons are easy to make. Makes good movies/videos, if done right. Most addons are good.
Cons: Community is ok but is still pretty bad. People can save your bricks and claim as own. Large amount children on forums and game.
3. ROBLOX
Pros: Make neat courses. Physics are ok.
Cons: Community is TERRIBLE. Dumb moderators. Laggy feeling, even at 50+ FPS.
I think I went a little overboard..