I enjoyed Chaos a lot. If I had to pick one that would be it. I wish it would come back.
I also remember Goobers being pretty well made but I could never get into it, I enjoyed farming more than the actual game. Same with Bravexia, although that always seemed more of a tech demo to me, since it wasn't actually fun but it was very technologically advanced (revamped monster AI primarily).
Shiva's first servers were pretty good too, very unique, albeit they didn't last very long and he was pretty greedy, making rich players literally invincible.
There were also two foreign servers I particularly enjoyed, namely the Naruto server by God Dew (Chilean if I remember correctly), despite me not actually liking Naruto at all, and the very first Dragon Ball server which was Polish; crashed all the time, was only online for like 4 hours a day and I had around 1000 ping... but it was still loads of fun.
I enjoyed Tibia servers a lot more back then, before real maps came into light. People actually put efforts into their servers (there were of course the download-and-run YurOTS, but they were the minority) and even simple stuff like having armor grant magic damage was something innovative. Those were the days.