With respect--and knowing nothing about you, what you like to shoot, or if camera weight matters--it's tough to advise but here goes. You've got three format choices, right? And I'm leaving out rangefinders in favor of SLR models.
First, I'd avoid relics. Old Yashica, Rollei, Mamiya, Minolta, Whatever-Flex TLRs can quickly become money pits unless you want to add camera repairmen to your circle of friends. You may get a gem but I'm off them after watching friends suffer--sorry, TLR fetishists.
645: Most like 35mm and the lightest. Mamiya 645(Super, Pro, ProTL--not the old heavy metal types) and Pentax 645 are good value. Great glass. Plentiful. Cheap and getting cheaper. Bronica ETRS(i)is probably the "best buy" MF champ.
6x6: Hasselblad. Bronica SQ(A, Ai, B). I have a Bronica SQ-B and love it.
6x7: Mamiya RB/RZ67. RB is mechanical; RZ electronic. RB67 Pro S and Pro SD bodies are the latest and best. RB is not petite but neither is it just a studio camera. Good affordable and plentiful glass in standard focal lengths. Rotating back(RB) feature on both models is killer. I have a RB 67 Pro S kit and shoot it more than my 645 and 6x6 cameras.
Pentax 67: Baby-sat one for a couple of years and liked it. Giant 35mm-like layout. Good lenses. No film backs(no mid-roll changes). BIG, heavy and surprisingly awkward off a tripod outside of studio work with strobes. Affordable.
Bronica GS-1:Probably the smallest, lightest 6x7 SLR system camera. Good lenses. Never a huge seller relative to the Pentax 67 or even more popular Mamiya RB/RZ 67 making it harder to find accessories and lenses.
Unless you have a home darkroom and/or a pro lab with good survival prospects, MF might not work for you. Sad but part of the new reality for us APUG inmates.