Team Fortress 2
I started mapping on Source for L4D (and eventually L4D2), then Portal, then dabbled into TF2 until we could modify the Alien Swarm SDK for Portal 2 capabilities. It's funny that, despite using the same engine, designing a map for those three genres of games is completely different. I would have to say Portal is the easiest to design for out of the three, but no less rewarding.
Designing a map for Versus L4D2 and competitive TF2 is when it starts getting serious and difficult.
I think the guy who made Barnblitz started making it as a custom map, but then was hired by Valve and finished it while working there. So both? That's what the rumor seems to be...
i love tf2 i am an okay medic and pyro, and i map, i have 1 map up on tf2maps.net
http://forums.tf2maps.net/downloads.php?do=file&id=4798
I play TF2 a lot, mostly playing Pyro, Sniper, Soldier! I got it back when it costed money dammit! 
[spoiler][SP] Alternate[/spoiler]
TF2 is one of the main games I play. Bought back in 2009 for the 360 and later got it that same year on the PC in the Orange Box. I consider myself an all around player. I play pretty much every class except Heavy and Sniper which I occasionally play. Although I think my best classes would be Soldier, Engie, and Medic.
I just tried to play TF2 with actual humans for the first time in a couple months. I'm afraid any kills I may have gotten were entirely coincidental.
Bots and humans don't play nearly the same, and I've played with bots for so long I forgot what an actual human-driven game was like. Guess I need to practice with actual people more if I wanna be any good...
I'm slowly realizing though that TF2 isn't as much my kind of game as I thought it was. I was raised on single-player games with very simple multiplayer functions if any multiplayer at all, and single-player games seem to be what I enjoy the most, even recently. Took me the last few years to realize this though.