I find this kind of question hard to answer since I play all classes on a fairly regular basis; as in, I play classes that have yet to be filled (no 'x' on team).
Class-wise, I think my most played overall would be either the Soldier or Engi.
Gotta go with the mad Scotsman. Can't resist an evening of good ol' head-slicing debauchery and fiery explosions which ignite sparks of joy in my eyes!