Probably 7200 (first to go that fast, I believe). There was a 5.25 drive made by the Seagate division in Minneapolis that develeped a drive called the Elite that spun at 10,000 - that sucker got really hot, but it was a nice piece of work. I worked for Seagate from '87 to '98, mostly on the Wren series (developed in OKC). But it was a division of Control Data when I started, Seagate bought us later.
Seagate had a crap rep when they first started, because all they made at first were stepper-motor HDD's. They bought Control Data's disc division (branded as Imprimis), to up their game (voice-coil actuators replacing stepper motors), and they phased out the steppers quick. That integration of Control Data was the beginning of Seagate's first swing of good reputation. edit - I was wrong about the Elite - it was earlier, and the first to spin faster than 3600, it spun at 5400. 3600 was the standard for a long time before that. I think it may have been a later 'Cuda that spun at 10K.