HPUX was without a doubt the worst UNIX imo. Solaris was great. IRIX was great. AIX was neat, if a little weird. But SSH'ing into an HPUX box felt like you had been transported back into 1979 or something lol
But in some sense, every high-performance platform back then was an abomination. Whether it was the variant of AIX on an SP2, the weirdly unique Irix versions that seemed to exist in each Origin system at each national lab, or the painfully slow fork/exec on a Cray T3E frontend system when compiling apps.
Irix, Solaris and NeXTSTEP are my favourite UNIXes.
The system was still expensive enough that we had a single tower for the whole class, we would take turns into the system, having prepared our samples on MS-DOS using Turbo C, with mocks for the UNIX system calls and conditional makefiles.