The Mamiya 7 is a great idea, it's probably the best compromise of portability and flexibility. It's got 6x7 quality, but you can shoot it like it was a 35mm camera. Either that or a GF670, less flexible with the built in lens, but even more portable.
So for me, certainly medium format, and something easy to hand hold. But it depends on location too, if I was going somewhere deep in nature, with a lot of time to spare, I'd probably want to bring along my 4x5 too.