I have played in all the serves and not everything i say will be true for some people but for me it is.
On the x10 i was doing arenas war/rdudu and we were going against 1500 fully geared people and winning. The queue for arenas were too long if you ask me. we were (300-350k)
On x100 is the most populated server. It's really cool if you like oneshoting people with 200k hp in bg's. If you are alliance the queue for bg is really long it says "100 allys/ 0 horde queued".
On the fun it's really cool if you like just doing arenas / bg's and hate lvling. Some of the old raid are closed i know about FL but for other i don't. The best thing in this realm is that you can buy pve gear and since half the set bonuses doesn't work so you can get BIS gear for your class.
As for all servers they are really populated and i thing is the world 1 private server for mop (i am not sure). There are some bugs and harldy any mop pve is working.Timeless isle is not sripted and you can see the bugged spells after the update top 5.4.8 in the bug tracker.