For me it's all about the role play, and being able to be someone outside of your normal persona
Bearing that in mind I would be going for Haunted Mansion or Tower of Terror as, from my own experience, these CMs seem to have a lot of fun getting into the role and interacting with the guests.
I'm sure the CMs on rides like TSM and Splash Mountain enjoy what they do - but other than loading guests into the ride cars there seems to be little scope for 'fun'