DR-DOS (back when MS-DOS was stodgy and you couldn't buy a retail copy, it added tons of features).
OS/2 2.0 (got my hands on a beta in 1991 and it blew my mind -- so much so that I applied for, and got, an internship during the launch). Preemptive multitasking, built-in support for DOS and Windows apps (gee kind of like Parallels/Fusion except built in), object oriented desktop -- IN NINETEEN-FRICKEN-NINETY-ONE. In many ways OSX/Win are still behind in features.
System 7.
Windows 2000 (this was when the NT family finally started to gel), it was rock solid.
OSX.