Hands down Rollei 6000 series. Interchangeable backs, faster darkslide (no dropping them like Hassie), pre-loadable inserts, auto sensing *with MotorDrive*, heck even the inserts are reversible so you don't have to swap around the spool, spring loaded spool release. No question. I've used all the others and if you never actually loaded and shot with a Rollei 6000 your jaw will drop when you see how fast it can load (maybe I should do a small video on youtube?). Faster than many 35mm from empty roll to ready roll.
Really fast. Did I mention Fast?