Your budget will largely decide what you get. I've bought and used a fair number of 120 based folding cameras (30?) plus another dozen 120 film "box" cameras (Brownie) and like using them. I've really come to love the ones from the 1920s and 30s. There are several ways to "group" them. First, are you doing to use a tripod or not? Cameras like the Pentax 645, Bronica--e.g. most that resemble the Hassleblad form, are generally best used on a tripod. Second, do you need a meter or can you use a handheld meter? Having a metered prism is going to drive up cost and limit you. Third is whether or not you really need interchangeable lenses or not. That will drive up cost. I started out in the 90s with a Bronica ETRS (645) system and liked it. Great lenses and system at the lowest cost. I later bought a Hassleblad with standard lens. I sold all that when I bought into digital.
Around 2010 I got bored with digital and bought a simple Kodak Brownie No.2 vintage 1904. I still have it and take photos with it! It's a lot of fun and I like the look of the photos. I also love the fancy "luxury" folders made in Germany in the 1920s and early 30s but those are a bit fiddly to use. I have three Rolleiflexes including an "Art Deco" Rolleicord. Those are very easy to use without a tripod and a lot of fun. Probably at their best when taking photos around a city as they're inconspicuous. Cameras made after WW2 will have a coated lens. There are a number of really good TLR cameras out there such as the Mamiya, Minolta Autocord, Yashica that are affordable. Another camera I like is the Voigtlander Bessa RF, a camera made just before WW2 and has a rangefinder, folding design, and takes 6x9 images. These are my most used medium format cameras, the ones I take on trips etc.: Kodak Brownie No.2, Rolleiflex MX, Voigtlander Bessa RF. I just don't need interchangeable lenses much. They add bulk and slow me down. I suggest you try a vintage Brownie No.2. They are inexpensive, plentiful, and a LOT of fun! Shoot Ilford FP4+ film and learn how to process your own. (Easy as making pancakes.)
Attached photo is a Bessa RF.
Kent in SD