When I choose to play a multiplayer game on TF2 (without manually picking a server), after choosing the map type, the game informs me that it is searching for the "Best Available Server".
To say nothing of often getting hundreds of results, I can do this, leave a server and start again and it will always go to one different than the one I just left.
So clearly it's not using a basic single-item criteria, and likely it's ignoring recently-visited servers, but on a whole, what actual physical qualities does TF2 look for when picking a "best server" for you to play on?