Favorite? Well not sure I have a favorite but there is one format not listed I pick up more than the others. That is 35mm half frame (or 'single frame', or 'cini-frame').
I have several 35mm half frame cameras including a nifty Olympus Pen F with a selection of Pen Zuiko lenses for it. With the ever rising cost of film it's nice to get 2X the number of exposures per roll.
As to my available formats, they are:
Minolta 16II, 16mm single perf., 10X14mm
Olympus Pen's, 35mm, 18X24mm
Olympus OM 35mm, 24X36mm
Various 6X6 for 120 film, 56X56mm
Kodak Tourist 6X9 (modifed for 120) 56X84mm
Flakey home built 4X5 (but I am a backer for the Kickstarter Travelwide, as of now 6 months late but getting closer)
One more, an almost finished, homebuilt 4X10in. pinhole that will used a curved piece of photo paper for a 120 degree FOV,
not completed, not tested.