Tough question, I have three.
Mizzou: I smoke anything and everything in this. Great for shorter smokes, and very lightweight. This gets the most use of the three.
Country Gentleman: This comes out to play when I'm settling down for a long smoke, usually when sat around the chiminea in the garden for a few hours. Again, anything goes in this.
Washington Bent: Previously dedicated to the stronger Lakelands like Ennerdale. Superb pipe!
However, if I was to choose my favourite MM pipe of all I've tried, I'd have to say the Bent Ozark Cherry. It's the current favourite of all my pipes. If I knew then what I know now, I'd have bought duplicates of this instead of other pipes.