10.0 was terrible, it was slow and lacked a lot of features that Mac OS 9 still had and looked terrible on lower res screens like the clamshell iBooks.
Yeah, Puma sucked. First really usable OS X was imho Panther.
At the time that these OS releases were happening, the company I worked for was still solidly using OS9. I knew what my problems were under OS9, quite a lot of money had already been paid out for software under OS9 and all the Mac magazines and articles I was reading then said that 10.x just was not ready to be used in any sort of production environment.10.0 is my vote. It showed a ton of promise over 9 - it really did feel like the future - but ultimately it was exceptionally slow and limited in functionality. 10.1 was a massive improvement.
I also took exception to the fact that I was forced to create a user account and a password when installing OS X. While OS9 had some elements of that, it didn't force it on you like OS X did. I wasn't happy with that.
By mid 2004 I was in another job and installed Jaguar there and on my own Mac. I was incensed to discover that popup windows and labels had not crossed over. Labels came back in Panther, but I'm still waiting for popup windows in 2024.
I was able to produce using Jaguar and Classic however, so by the time Panther came along a lot of my headaches had been resolved. Panther takes the win however for stability. Solid OS.