Least favorite is I because it's still kinda weird. The series didn't really find itself until II, IMO.

I never got why III is so hyped. The story kinda fell apart in my eyes on the way, and the end part seemed really rushed.

IV is my favorite of the, let's say, "traditional" BoF games. It really had a great story with a strong cast of characters, and a very nice world.

I adore Dragon Quarter for how original it is and how every time you play it's different. I can see why people didn't like it too much if they were expecting more of the same, but it's still a damn great game.