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Here are my 5
1. Anarchy Online - Even to this day I find myself relearning this game every time I go back to it, very awkward but cool
2. Eve Online - This one was a tough one for me and as bad and maybe even more than AO, definitely a deep learning curve.
3. Planet Arkadia - Just a bizarre and different experience I have ever come across in the world of mmorpgs.
4. Face of Mankind - As many times I have tried this game within the last decade, I find myself probably going back to this one to check things out. I never got far into it, but what a crazy experience every time I did log on.
5. Ultima Online - Ahhh, A fantasy game that kept me busy for many years, even now, however I find myself spoiled today with graphics, however I find myself just checking things out in Britannia now and then as well.