As others have noted, it really depends on the kind of shooting you enjoy. I've had a Rollei twin lens for years, and I enjoy the relatively light weight and deliberate shooting style. I also have a Zeiss medium format 6x6 folding camera, which really is lightweight and gives almost the same operational speed as a 35mm rangefinder. It's a delight for travel if I want to bring back high quality negatives with a camera that fits into a coat pocket. There are many other choices available in the folding camera range.
I finally got a Hasselblad 500C just over a year ago, and immediately fell in love. I now have lenses from 40mm (about equal to a 28mm on a 35mm SLR) to 250mm. I have an on camera reflective meter, and a metered prism for eye level use. It's the most versatile of all the 6x6 cm systems, and one of the lightest. Bronica S is a lower priced alternative, as is the rather noisy and less well made Kowa 6 series. Mamiya RB and RZ 6x7 format cameras are a bargain, and have excellent optics in a wide range of focal lengths, but they are a lot heavier and bulkier than the Hassies. Rollei SL66 series is also fantastic, but difficult to repair and it's getting harder to find lenses and accessorites. It's a beautiful camera, though. I don't care for the relatively lower build quality of the Russian Kiev models, but they are certainly affordable.
There are many others, the Mamiya 7 and Fuji groups of 120 rangefinders, many slightly lower quality TLRs (Yashica, Minolta, Meopta, to name a few...), and the fabulous interchangeable lens Mamiya TLRs. The Pentax 6x7 SLRs are also excellent, but they are really big and heavy - I sold mine.
The X factors in this format are generally price and availability. Hasselblads, Mamiya 7s, and several others have taken off in the past couple of years, and you have to shop carefully and take time. Others are starting to head in the same direction. Once you make a decision, you need to commit to that, both in terms of time and money. It's always a compromise - speed vs. negative size, lens selection vs. price, etc. I'd almost advise an early experiment with a TLR, Cheap S:R (Kowa), and some kind of rangefinder (although you'll probably only be able to afford a folder as an experiment. Once you get a preference for shooting style and type, then you can go all in on a system or high end model.
Good luck! I shoot everything from digital to 4x5, and the three I mentioned above give me by far the most pleasure.
Andy