http://www.lomcn.org/forum/showthread.php?86448-Mir2-Server-Status-List
Chronicles, Arcadia and FiveHeroes are all Mir2 servers worth at least trying and all have 100+ users. Chronicles uses their own ACEm2 files, Arcadia uses heroes? files and FiveHeroes are using the more recent c# files aka Crystal files.
Imo if you want to be able to compete sooner rather than in a few months of grinding, you'd be better off joining FiveHeroes as it's the more recent of the 3 servers. However if you're looking for more stability i'd recommend Arcadia or Chronicles as I simply cannot ever see them closing. They are both great servers. I'd tend to lean more towards Chronicles out of those two, but that's just me.
As for Mir3 servers honestly I can't say much but in terms of population nothing comes closer to DarkSide.
edit: ****ing ninja coders these days.