I kinda like the farm area because you can go around the yard, into the barn, upstairs then back downstairs it's probably the easiest of them all besides the tunnel. I've heard from friends who always do the tunnel that it's the best. I've tried bus depot for awhile and it wasn't that bad either. It's really tight knit and sketchy when you have to deal with lava cracks and so forth. But there are so many places to run circles on the map it's insane :p
I like rape training on Tranzit, specifically at the gas station. I'll buy an MP7 from inside the Diner near the Sleight of Hand machine. It's not too difficult in my opinion and the trains are very efficient for gaining large amounts of points. Especially if you melee the zombies with the galvaknuckles at low rounds.
I love the Tunnel, I always do good there. If not the tunnel then I really prefer Town if it's just me because I hate having a teammate there. I've also clutched it for our team at Farm but like someone else said, the bus can get in the way and mess you up a bit.
I either run a train in the Tunnel, or at the Diner, not many run them at the Diner but there's a really good spot there, and enough room to not get trapped.
I would say Town but there's no weapon that you can keep getting there. I would have to say Diner or Tunnel because they both have a weapon that you can keep getting ammo for. Even upgraded.