It's 4,5x6 on 120 film for me. I prefer 6x6 instead. Getting a few more exposures out of a roll is no reason to use smaller formats, since usually 12 frames are just enough and I can still crop a little, when the square format doesn't fit a subject. When I need more frames and no great quality for large prints, I'll use 35mm film or even 35mm half frame when I'm in that shoot-everything-no-matter-what mood.
11x14. I've got a complete camera, lens, and glass plate holders that can do double duty with film, and even a partial box of 11x14 film. But it's such a pain in the ass to use it, the camera sits around just looking pretty and not doing anything. My 5x12 and whole plate cameras get more use, and they're far stranger sizes.
35mm, but the Hasselblad is feeling ever increasingly more abandoned. I've decided to keep it because the prices of used medium format digital backs for it are beginning to approach the doable range on Ebay.
I answered 35mm, John, but you are right about 4x5. I have not used my 4x5 since the period between when my 5x7 was stolen and when I got a 8x10 using the insurance money (1995).
120 (6x6 & 6x9) & 4x5. I have all of the gear but have been concentrating on 35mm for several years. Most of it due to lack of reliable transportation. Lots of 35mm subjects withing hiking distance of home that don't really justify the other formats.
That is changing soon though. A rebuilt transmission for my Jeep 4x4 is not far off. Took a long time to save up the $2200 for it and some transfer case parts. Then I can get to some places I've been itching to photograph with the other formats.