If you are patient, you should be able to get most non-rangefinder MF systems with a standard lens for that price (even Hasselblad! although it might be a longer wait).
It really depends on other needs that you may or may not have. Do you like to travel? Do you want more lenses eventually? Do you use a flash system? etc...
The four I would recommend looking at are not the cheapest, but I do think they are the 'best' for most users:
Hasselblad 500c/m (or 500c, can be found a little cheaper)
The classic. Personally my most used system. It's really a swiss army knife - 6x6 and 6x45 backs available, all lenses excellent, flash sync up to 1/500s, small and portable body, and in production for an extremely long time - resulting in plenty of backup bodies and lenses, and many available technicians. The system is 100% mechanical, no batteries or electronics. the 80mm 2.8 is a brilliant and compact lens. Speciality lenses (I'm still waiting for a reasonably priced 100mm 3.5...) and accessories can quickly get extremely expensive, though. The older C and CT* lenses are much cheaper than the newer CF/CFi lenses for 95% of the performance. The 120 5.6 S-Planar is an absolutely killer optic that can be found for low prices.
Budget alternative would be a Bronica SQ, a very similar system but without the sexy body/lens design. the Bronica SQ can be found for a song in comparison to a Hasselblad kit - I expect repairs might be harder as it involves electronics and they aren't quite as ubiquitous, but they are cheap enough that it should be easy to replace anything for the foreseeable future, even if repairs are difficult.
Pentax 6x7
A beast, often compared to the handling of a 35mm SLR on steroids. Killer lenses and extremely good prices on lenses (bodies a little pricey compared to many) - honestly, I think the build quality on the Takumars exceeds even that of the Zeiss glass for the Hasselblad. If you want long or fast glass on a huge negative, they are a great option. The standard 105 2.4 is a brilliant, brilliant, brilliant lens.
Mamiya RZ 67
The apex if you work primarily in studio/on tripod. Rotating back, bellows focusing, absolutely top notch lenses. For the quality involved, the prices today are a joke. If you want flexibility and top quality, I think you'd be hard pressed to find something better - although it lacks the elegance of the Hasselblad or the bomb-proof feeling of the Pentax. It would be my top choice if I knew I was going to be working primarily in a studio setting, but as a do-all/go-anywhere solution I think it would take third place.
Rolleiflex TLR
If you want high quality and high portability and don't need interchangeable lenses and backs, nothing beats a Rolleiflex. It is a much more limited system, though, and prices can get high. If you go this route, I would reccomend buying a body you have the ability to return, and checking it's focus at all distances - sometimes the front standard can get knocked out of alignment but can be hard to see unless you are shooting wide open. Prices very widely, but an older 3.5 model or a non-pristine 2.8 model can be found under $500 (I got a 2.8D in VG condition for $300... again... patience pays off).