It really depends on what you want to shoot, how much gear you want to carry, and the snob factor...
I've owned or used probably 50 different medium format cameras over the last 20 years (it helps to work in the photo trade-I've worked in retail, as a photographer, and as a camera tech, plus I collect cameras) so I've had reasonable experience with various cameras.
The bottom line is that pretty much any medium format camera that is working well will give you good results...what I've found is that it comes down to is personal choice based on negative size, and the type of camera you get along with..
Some rough observations:
6x4.5 format - is a great entry to mediium format - and are great for street shooting and portraits - I've used Mamiya, Bronica and Pentax SLR's - all are good, although for street shooting I would go for the Pentax because you have auto and manual exposure built in to the body, the built in winder isn't too noisy, and the lenses are super sharp (not to mention cheap). I've also used the Fuji rangefinders - love the one with the 60mm lens, and the folding one is a great all around camera (although you really need to check for pinholes in the bellows - most I've seen have had them..)
6X6 - lots of choices - TLR's (Rolleiflex and Rolleicord, Yashica, Bueatyflex, Autocord) etc with fixed lenses are all quiet, reliable and sharp (so are the Seagull's if you find one that's been tweaked), and with the waist level finder you can focus discreetly (although I have to say some of the viewfinders can be a bit dim - you may need to get the camera serviced, and replace the focussing screen or mirror to get a brighter viewfinder). Mamiyaflex's are great - but heavy!
6x6 SLR's are great, but they are heavier, and not as quiet...Hasselblads are a great camera, but the lenses can be slow to focus as you have to move the focus ring through nearly 270 degrees to go from infinity to close focus - can be frustrating if you are trying to focus quickly...Although if you can pick up a old Kowa 6 (not the 66 with the interchangable back - the backs could be a bit of a pain) then I think you will love it - the results on par with a Hasselblad (I've shot both side by side), just be aware they do go off with a bit of a thump (just like a Mamiya RB/RZ or Bronica EC/S series..)
6x6 rangefinder - now we're talking....very under-rated by people (and I'm not just talking about the Mmaiya 6) - look for a Zeiss Super Ikonta (coupled rangefinder) or Zeiss Ikonta, or one of the Agfa Isolettes (easiest way to tell if they are any good is to get one with a 1-200 shutter - they always seemed to put a good lens on one with a full range of shutter speeds). I've also had great results form the Chinese Seagul 203 cameras, and also the Russian Iskra cameras.
Even though most of the Agfa's (and the Zeiss Ikonta's) don't have a coupled rangefinder, you soon learn to guess your distance accuratly, and I defy anyone to tell the difference between a photo from one of these and a Hasselblad in the final print..
6X7/6x9 - The Fuji rangefinders are great - quick to focus, and sharp lenses. Same for the Mamiya 7, and you get interchangable lenses..Mamiya press was a great camera too, but very heavy. If you can find one then the Graflex XLRF was a great camera - 6x7 rangefinder with interchangable lenses (although youwill be hard pressed to find accessory lenses) - super sharp lenses, and as they were made of magnesium (I think) VERY light....
You could also consider a folding camera - Zeiss Ikonta/Super Ikonta - they produced stunning results. My personal experience with the Russian Moskva hasn't been good - most had wobbly beds, or the rangefinders were out of adjustment (and not easy to fix)..Same goes for the Voigtlander Bessa II - great camera, but watch out for a loose/wobbly lens bed...however if they are tight then they are one of the nicest cameras ever made....
6x12 - why not, if you are into panoramas? I've used some of the scale focussing cameras fitted with 90mm large format lenses, and they are super sharp....I've not had much luck with Noblex's though....
I suppose the bottom line is that for a couple of hundred dollars you should be able to pick up a decent camera...remember it's a bit of a learing curve, and there are some cameras (no matter how good they are) that you just won't get along with.......
Have fun!
I have been shooting digital for few years. My focus is portrait and PJ/street type of work. Lately I got a 35mm film body to work with my Canon lens collection. I love it very much because I don't need to care about white balance anymore
Seriously I love the dynamic range, grain and imperfection of film. So I would like to get a medium format set to do some portrait work. 
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