It depends on the experience you had on each server.

Classic: I love Guild sparring and last year was my first time participating in the GST. Fun experience overall.

Era: My home server... What I love about era was the events and sparring back before the bar. I was there that day the bar was released and so I went to the underground spar only to see how destructive the gun was! Seeing a guy go 50-0 against m4 user terrified me. My crowning achievement on era was my 1k spar wins. I was like in the top 100 on Gamecenter as well. One day though I'll get an npc in one those shops that will be introduced.

Zone: I haven't played zone much since its release, but what I like about Zone is the 360 shooting. It is so amazing and the guns look so awesome! More to come as I'm playing zone more than era.

Delteria: I haven't played it more than 10 hours, but I loved dungeons and how well developed it look. When this game comes out for ios I can see it getting a high playercount when more updates roll out.

Zodiac: I loved how you could choose your own race and don't get me started on how awesome the weapons look! I didn't play this long though :[