I owned the GS645 (not the wider S version but the standard 75mm) and liked it a lot once I got used to the ergonomics and handling. And it took great photos. There are actually three cameras in the GS645 series. The one linked above at 75mm, the S at 60mm and the W at 45mm. I guess you just need to decide which focal length suits you best. The 75mm standard model is the only one that is a folder. Some like that and others don't. I ended up selling mine to help finance the GF670 which is clearly a superior camera in just about every aspect. But it is very costly as well.
If you want an exchangeable lens rangefinder in 645 I think the Bronica is about your only option. (Someone will probably correct me if I'm wrong).
If you want an SLR there are lots more options as mentioned above. Bronica, Mamiya and Pentax all have 645 models.The Mamiya is the most affordable with a huge lens lineup and several different body models to choose from, but the Pentax is probably the nicest to use, even though it isn't a fully modular system like the others. I know your first reaction to the Pentax was negative so I won't try to change your mind, but I own and use the Bronica ETRs, Mamiya 645 ProTL and the Pentax 645n and my favorite is the Pentax. It is the classiest looking to my eye and has the best ergonomics.
If you want to shoot 6x6 square frames the Mamiya 6 that you mentioned at the beginning is a fabulous camera, and is currently my most frequently used MF camera. But alas, the price is a bit steep. Other rangefinder models are mostly limited to older folders but they can be a bit of a crap shoot in terms of quality. Then there are the SLR options such as the Bronica SQ series, older Bronica focal plane models, and the nicest but most expensive option, Hasselblad. Finally, as others have suggested, a TLR might suit your needs if you don't mind ground glass focusing and the boxy shape.
If you want to go up to 6x7, the Pentax 67 (4 different models available), Mamiya RB or RZ (several models of each), and Bronica GS1 come to mind. Of those options the Bronica is the easiest to hand hold if you give it a speed grip and AE prism finder.
If portability and compactness are a prime concerns the rangefinders almost always win out and 645 is the smallest medium format available. Among rangefinders, the folders are the most compact when not in use so the GS645 might come out on top. But unless you actually like and enjoy using the camera none of them will give you a positive experience. So try them out, and find what suits you best! Good luck!