If you want small then your best bet is a folder, or put up w/ 6x4.5 (which is still a huge negative) and get a Fuji GS645s. It has a meter, can take 120 and 220 film, and is light and small. It also feels and looks like it's made out of cheap plastic, but if you can get past that it's a great shooter.
The folders that are fastest to use will have coupled rangefinders. Any of the Zeiss Super Ikontas come to mind, as well as the Welta Welturs. Both of these generally have excellent bellows, and I would look for a Welta w/ a Xenar or Tessar lens and a Zeiss w/ a Tessar as well. The Novars image very well too, especially past f8, but not as tack sharp as the Tessars under that. The Weltas will give you sharp photos even closeup and wide open because the whole lens focuses like a LF camera, not just the front cell. They have 2.8 lenses, which is plenty fast for MF.
If you are willing to use a camera w/o a coupled rangefinder than the Bessa 66 or Perkeo are good shooters, and the Agfa Isolette w/ Apotar or Solinar can be picked up very cheaply, but they don't have the sturdiest of bellows. If it were me and I just had to have 6x6 it would be a Welta or Zeiss Super Ikonta. What do I shoot though? I don't need the RF, but I do have a Welta w/ Xenar, several Agfa Isolettes w/ Apotar and Solinars, and the crown jewel, a Bessa 66 w/ Heliar lens. The Heliar is worth buying a camera for in itself.
I'm not exactly sure what Dante means about the lenses performing to modern standards. My 50 year old Bessa II has a 105 Heliar and it's the best lens I've ever used. I would be more concerned about the modern lenses performing up to the standards of the Xenars, Heliars and Tessars of yesteryear.
