Why do you want to shoot medium format? What are you after that 35mm is not giving you?
Do you want an SLR, a TLR, or a rangefinder?
Do you want interchangeable lenses? (If you answered TLR above, this leaves the Mamiya C system and the 6x6 square format as your option and you can stop here.)
Do you have any physical trouble carrying and lifting slightly heavy loads (Shy away from 6x7 SLRs if you do, and look at SLRs with smaller frame sizes.)
Do you want interchangeable magazines? (Scratch Pentax 6x7's and compact rangefinders such as Mamiya 6's and 7's if you do.)
Will you shoot it on a tripod or hand held, or both, how often, and in what circumstances? (If handheld often, lean toward systems with faster lenses – f/2.something.)
Are you technically careful enough to bother getting the ideal exposure, or do you prefer to just point, read the in-camera meter, and shoot? (How important is the meter, really?)
I'll ignore frame size for now, as any frame size can be cropped, and quality will still be better than 35mm. I'll also ignore price for now, just so we can determine what the OP actually wants in an ideal world first.