L4D is probably the only worthwhile one
TF2, we would have to steal traffic from Gibs/NR, and those guys have a nice and packed TF2 servers with a strong following, plus very few TD players are crazy enough about TF2 to bother moderating.
DoD, dont even get me started on the "realism" and "roleplaying" zeal "popular" servers possess, if you want to have a solid DoD server, you would need a solid group of players pretty much dedicated to it. The sad part is, all of the decent servers that i ever played on, are like these weird christian shit where you cant swear or insult people, but killing ze germans seems to be okay. Also when you TK someone, you have to wait and see if someone says "i'm sorry". If you slay or punish and even if they said sorry, the Admins get on your ass.
it really is a headache...
L4D seems like it would require the least amount of maintenance
and all the other ones are not worth mentioning