I liked pretty much anything in the early days because it felt something like a small exclusive club, and a global discussion network like Usenet was just so unique. That experience of helping someone across the globe while sitting at home was just really cool.

The sweet spot technically for me was probably later Solaris with things like zones and dtrace that nobody else had. The popularity of Solaris also made it much easier to find help from other people, Makefiles that just worked, SSL accelerator boards, etc. Ask anyone that worked with zones, for example, and we find all the buzz about docker and linux containers quite funny.

I probably hated AIX the most. It was tantalizingly close to other Unix implementations, but with enough differences that things would sort-of work, but not really. To be fair, there was a "right" way to do things that worked, but I was naturally trying to use knowledge I already had. It was also the platform most likely to bomb out when trying to compile open source software. Lots of tweaking makefiles, environment variables, compiler flags, etc, to get things to work. They had a sysadmin tool called "smitty" that I particularly hated.