Ennnh, it depends on what day and what sort of project I'm contemplating [ya see where this goes...]. If I look at the last couple of years, I might say 6x6 -- 1:1 aspect ratio -- but most of my teenage and early to middle age adult years I shot 35mm (well even before that there was 6x9 in my Brownie Target Six-20) so I cut my teeth on 3:2. I also stumbled into some 4x5 work for a while but never did a lot with it, partly due to expense and lack of a good enlarger. I still own
every camera I've ever bought, though two are rather defunct, and a third needs a CLA on its shutter. [Speaking of GAS, and hoarding ....

]
Since my "sudden" retirement in 2002 and a rekindled interest in the darkroom, as wedding photographers have abandoned medium format for zapped bits, I've acquired a significant stash of Bronica gear which I consider my go-to for "serious" work (whatever that is); that's 6x6. But it's pretty hefty to carry around, so I added a Perkeo II folder (6x6, 80mm Color Skopar) for more casual and/or travel situations. And then I added an Ercona II, a 6x9 folder (105mm Carl Zeiss Jena Tessar), mostly for cool factor. It's awesome to look at that sucker with that 105mm f/3.5 on the front! Then because I could, I added a Yashica Mat 124G that's 6x6, is considered benign by people on the street, and works nicely for infrared as one isn't trying to look through an 89B filter to compose and focus.
So I guess I kinda like 120 roll film. Currently the Bronica has the most shots, followed by the Perkeo (but I've also had them longest in this post-retirement period).
But guess what -- Tuesday I was out with my 8x10 pinhole camera playing with x-ray film!
(And there's a Minox B around here if film becomes more available -- I recall that's about 11:8!

)
I still have a bunch of Canon FD 35mm stuff which I occasionally use, but admit I actually do a lot of casual shooting with "other technology" and out of several formats find the 4:3 probably most to my liking.
Good grief [sobs, grabs Kleenex] I even have recently sunk [sniff!] to owning an iPhone 5c -- that's 4:3. (Although it wasn't acquired for photography, but naturally, I've tried it -- surprisingly good -- on stationary objects in daylight ...)
Um, what was the question .... :munch: