My preference is the largest bent I can get -- like the Peterson House XXL Pipe. I am almost always working while I'm smoking so I like a pipe that I can clench, that doesn't put a lot of pressure on my teeth.
I've always been amused with the size of this Calabash that Col Lanta lifts from his...
The Peterson System mouthpiece retails for $25 in either P/Lip or F/T. Currently, you can get a M/P in either that fits 301, 303, 304, 305, 313, 314 & B42