Mamiya TLR, C3 or C220 or maybe even a 330, with one or two lenses. Or one of the many 645 system cameras. Or Bronica SLRs. Rolleicord or old Automat, Yashicamat, Minolta Autocord if you don't need interchangeable lenses. A high-quality folding camera with a rangefinder, possibly, but you're then dealing with possible bellows issues and fairly slow handling.
If you're moving to medium format specifically for image quality and want faster handling than your 2x3 Graflex, I wouldn't recommend using an antique or any of the box or toy cameras. Not that they're not great--I own several and shot many of my favorite images with a few-but they're not going to be the same as a camera that offers relatively fast, precise, and reliable focus and exposure controls.