Most private servers are about 30% complete, with MANY Missing mobs, MANY Bugged quests and NPC,s do not EVER believe it when they brag about updated Aion and quests and stuff, its all bullshit.
Also never believe things like x50, x80, it doesnt work how you would think it does, it will be x50 for 1-10, then go 25 for 10-25 then go down to like x15 or x10, the people running these servers have not configured their java files correctly for progressive EXP Boosts, or they just have extremely bad maths skills.
Infinite Aion is good if you've not played retail, as its just the same as retail pretty much but with few bugs and missing things here and there, it is however the most popular with most online.
Advanced Aion is mega bugged, not many people online ever.
Chronic Aion is also mega bugged and not many people online.
AionOcean is bugged and never anyone online, I was on for 2 days and was the only person on the entire server.
Many of these servers fail to prevent opposing races getting into starter zones, for example Asmodians can teleport into verteron and poeta and its annoying.
Many say "Abyss Points" but do infact have a completely bugged system, for example: No mobs give AP, only people, some have limit of 40-50 AP per kill, 200 per death, some have huge boosts but if you die you lose it all.
I've yet to find a decent private server without many bugs and with good AP systen.
I'm currently trying Atomix.
If you want to test the best your best of going here and trying the top 5:
There is probably more but some havve different top 5 than others.