I cut my MF teeth on Rolleiflexes and Rolleicords, but have owned the following SLR systems and used them to various degrees:
Bronica SQ-A
Bronica ETR/ETRS/ETR-C
Bronica S2a
Pentax 645
Mamiya RB67
Hasselblad V system
I have "slimmed down" to the Pentax, Hasselblad, and Mamiya. All three serve different purposes, and don't see myself getting rid of any of them.
The RB67 gives probably the best technical image quality of the three, but I don't use it that often. I have a range of lenses from pre-C to K/L, and while the K/Ls are a tiny bit more contrasty, overall I don't see a huge difference between them and even the earliest lenses unless the lighting is tricky. Between two bodies, 6 lenses(50mm-250mm), and a whole bunch of accessories I probably still have less invested in the complete system than I do my 4-lens, two body Hasselblad system. That includes some oddball stuff, too, and even a metered prism(which weighs over 2lbs by itself). Someone asked if 645 and 6x6 backs were available for the system-I have a 645 "Pro SD" back(these don't have foam light seals like the earlier pro and pro-S backs, and almost all backs that have not been refoamed will leak). I'm not sure if Mamiya made a 6x6 back, but Graflex-branded backs will fit and work(and those are available in 6x6). It's been a while since I really looked into it, but I think you want to avoid knob-advance Graflex backs as they can have film flatness issues from modern film being too thin. With that said, the whole camera is such a beast that I have a hard time using the smaller sized backs on it-if I'm going to go to the trouble of hauling it out, I'm going to get my 10 shots of 6x7 on a roll. Aside from the weight, it's also worth mentioning that it can be clunky to use since you have to cock the body and wind the film separately, and the film advance lever requires two strokes. Interestingly enough, in LF a lens mounted in a Seiko shutter will often bring less than the same lens mounted in a Copal shutter, but I've found that the Seiko shutters in my RB lenses keep ticking away without missing a beat. I am particular about storing them uncocked(just like my LF shutters), but I don't have a service history on any of my lenses and all within a half stop even at slow speeds(and about as good as you could hope for a mechanical shutter at high speeds).
The Pentax to me is honestly a lot like using an overgrown 80s 35mm SLR. It gives you even some primitive AE, although the focusing screen is dim and the controls can at times be clunky. As is a common theme with MF systems, you really can't complain about the lens quality. One of the things I've been spoiled on and consequently don't like is that the backs are not interchangeable, only the inserts. You can stock up on inserts and keep them handy, but you can't swap film types on the fly. I find this to be a limitation-you may not.
I resisted getting a Hasselblad for a long time, but there's a reason why it's the metric by which other MF SLRs are measured. The system has quirks out its rear, and heaven help you if you do things out of order. All of my lenses are the old chrome style "C" lenses, and I have FLs ranging from 50mm to 250mm. Even though my lenses are not T*, I can't really find anything to complain about optically on any of them-there are no "bad" Hasselblad lenses-just some that not as good as others. Ergonomically, there's a fair bit to dislike about the C lenses-they have scalloped metal focusing rings that turn probably 330º from infinity to the closest distance. Despite the seemingly endless travel, though, they range from "fairly stiff" to "I'm going to end up with blisters after using this lens for a day." I actually had my 150mm out yesterday, a lens I haven't used much, and reminded that the focusing ring is a lot easier than the 50mm and 80mm I use a lot more often. I've been told that a service can only do a limited amount to fix stiff focusing-a set of quick focus handles is a lot less expensive and works a lot better, but is also a problem if you want to use any sort of flash(the handle gets in the way of a sync cord attached to the lens). The Syncho-Compur shutters don't have the best reputation for longevity, although mine are all good aside from dead slow speeds on the 80mm. Hasselblad made more accessories than you can imagine, and it's easy to go wild on them. Some, though, like 120 backs, can cost you a fortune though. My primary body is a 500C, and I replaced the screen myself with one from Rick Oleson(the C/M added easily interchangeable screens, but the Acute-Matte screens can cost half or more the cost of a body, and the non-Hassy ones are either even more expensive or are terrible). I also have a 500EL/M, which actually makes a great secondary body-they are less expensive than manual advance bodies(I didn't pay a lot more than a what a stand-alone WLF would cost for mine) and there are readily available adapters to run them on a 9V battery. They can seem a bit unwieldy with an 80mm lens, but actually balance nicely with a physically larger lens(both my 50mm and 150mm make almost a perfect, if heavy, package). None the less, I wouldn't necessarily dive right into a Hasselblad as a first MF SLR, even though mine is now my default MF camera to use unless I have a good reason to use something else.
The SQ-A was my first MF SLR, and I still have a soft spot for the camera. I'll also kind of group the ETRS in with these comments, since they're quite similar. I primarily used both bodies with a speed grip, which made them handle nicely without a lot of extra bulk. The speed grip gives you a lever to advance the film(two strokes) and also a hot shoe-something that I found really handy for when I wanted to use on-camera flash(I often find myself using my giant Metz 60 CT-4 when I want a flash on my Hasselblad since the speed grip I have for it is left handed, only has a cold shoe, and isn't anywhere near as nicely done as the Bronica grip in my opinion). The electronic Seiko shutters of course need a battery to run at anything other than 1/500, but otherwise never go out of time and can go for 8 seconds. As usual, the lenses are excellent. Leaky backs were a constant thorn in my side. I didn't mention this in my comments on the RB67, but the Bronica and RB67 backs both are also a real pain to reseal in my experience, and I bought kits from Jon Goodman with his excellent instructions and pre-cut foam. If a body dies, forget trying to get it fixed-just buy another. I hate having that attitude, but if you can even find someone to work on them you'd pay a whole lot more than a replacement body.
The S2A looks great on paper, and there's a lot to like about it. The system does include one of the few legendarily bad MF lenses-the 13.5cm Nikkor, which really is a 35mm lens that just happens to cover 6x6. Mine wasn't AS terrible as some would have you think, but at the same time I had a mix of both Nikkors and Zenzanons running from 50mm to 250mm(a common theme of my MF systems, if you haven't noticed) and it did stand out as being worse than the others. The lenses are nice and lightweight since they don't have a shutter or even a focusing helical, but mounting/unmounting is a bit of an interesting process. Also, this range is notorious for degrading foam that causes focus issues-I had bellows but never really got satisfactory results. Mine was okay at more "normal" distances, but of course macro distances are very demanding of focus accuracy. The film advance gears are also a known weak spot, although the S2a is SUPPOSED to have fixed those. Overall, though, even though the camera has some interesting features the way it gets there is a Rube Goldberg nightmare. There's a LOT that has to happen in sequence-specifically the mirror dropping down(not flipping up), the blinds covering it and the focusing screen, and then lens stopping down and the shutter opening before it all goes in reverse-unlike more conventional "Hasselblad-style" MF cameras, it has an instant return mirror. You have cables and so forth all over the place to make all of this happen. The cameras make a ferocious amount of noise both when firing and when advancing. I could feel a lot of vibration from it when holding-partially damped out by the weight(it weighs a lot more than any other 6x6 I've used, including the SQ-A and Hasselblad)-but it's worse than even an RB67 to me. I really think most of the vibration comes from the shutter and not from the mirror. Also, the 1/1000 top speed SOUNDS nice(especially when you get 1/500 on leaf shutter 6x6 cameras, and 1/400 on the RB), but in reality the relatively long curtain travel time(1/30) means that the shutter isn't so great at stopping action or even motion blur. The 1/30 flash sync can also be a real hindrance.