Almost all of my MF work is done in 6x6, in large part because the square format provides a break from and contrast to the rectangular 35mm format I use for the majority of my photography.
I have equipment to shoot 69 and 68 but moly 66 by choice. Hasselblad, Rolleichord, toy cameras and pinhole cameras. The GX680 is n9o my choice for candid or backpacking
Favorite? 6x6. I first shot with the format eons ago, with a borrowed Rollieflex. I loved the challenge of trying to compose with the square (I'm a full frame shooter; I don't crop, irrespective of the format). When it came time to buy into a system, I looked into a few (Rollie, Mamiya, and Bronica) before settling on Hasselblad (2 x 500c/m with 50, 100, 180, and 250mm lenses). No regrets!