Best free servers by patches.

[TradePrince]

Best 1.12.1 server: Feenix wow (wow-one.com) 1600 players + online on peak times. Feel the pre tbc again!
Best 2.4.3 server: Feenix wow (Archangel, wow-one.com) 1100 players + online on peak times.
Best 3.3.5a server: wow-tournament.com aka abyssal-wow.com, 2500 players + online on peak times. Hydra & crew plays tournaments there.
Cataclysm server: I personaly prefer retail.
Note: I don't own those servers, those servers are ones I used to play/I still play.

[Stige]

arena-tournament.com is by far the best private server there is imo.

[Xyborg]

I played on Molten WoW briefly and it was pretty good.

[stoneharry]

arena-tournament.com is by far the best private server there is imo.

Hurp durp, let's just advertise our own servers and say that there the best, just because they are without any other reason.

/closed because of people like quoted.