Magnum, boomer bile, and the combat sniper rifle. I typically enjoy carrying the BB because it's more effective than a pipe when it comes to gathering Hordes. It's saved my ass in Hard Rain more than once. On Expert, the combat sniper rifle takes about sixty rounds (~reloads) to kill a tank by itself. That's the major reason I carry it.
I carry the DE because it's the most OP weapon in the game.
I base my weapon choice around this. http://hellforge.gameriot.com/blogs/Hellforge/Left-4-Dead-2-Weapon-Effectiveness-vs-Tank
There's always a tank. Always.
HARD RAAAAAAIIIIIN!!!