I am posting my personal list here, of servers that I have played on/tested. I would like others to add their personal favorites and explain why they think they should be added to the list. You may also disagree with one of the realms I posted, but be sure to be reasonable.
Also please keep the fun servers out of here. Rates also shouldn't matter, however bugfree content and especially proper class balance (spells/talents) are a must.
There are 3 rather big and serious Classic WoW servers:
Vanillagaming.org
500 People
Every instance including Naxxramas scripted
Donation/Vote rewards, nothing overpowered - needed to keep the server up
Working pathfinding system
XP rate x15, all other rates kept low for server economies sake
Worldofscriptcraft.com
2000 People
Heavy focus on PvP, some old Warcraftmovies legends rerolled there
very strong community (see above)
Instant level 49 (if wanted)
XP rate customisable from 1 to 5
Every instance scripted
No donation/vote rewards, donations are welcome though
Pathfinding working
Easily enough players to raid/pvp in whichever timezone you are
wow-one.com
2500+ People
Balanced focus on PvE/PvP
Donation/Voting for balanced rewards
XP Rate x12 or x1 or instant 60
Pathfinding working
Every instance scripted
Easily enough players to raid/pvp in whichever timezone you are
The Burning Crusade unfortunately does not have a lot of really good servers
wow-one.com
600+ People
Every instance scripted
PvP working almost all the time
Progressive content, currently T4/S1
Donation/Voting possible for balanced rewards
Balanced focus on PvP/PvE
wowen.hellground.pl
2500 people
XP rates x2
Balanced focus on PvE and PvP
PvP/PvE regardless of the timezome you are in
Every instance scripted, progressive content T5/T6 as of lately
Pathfinding working/in development
LastWoW.com
1500+ people
XP rates x20, honor x2, arena x2
Balanced focus on PvE and PvP
However instances not scripted as well, players focus on PvP
Arena-tournament.com
250 people
Instant 70, S3 equip
Most bugfree server out there, most known for pure arena players
Donation for rewards, nothing overpowered
Originally launched by Kisha and me years ago, if your personal focus is on arena, check it out
I DO NOT HAVE ANYTHING TO DO WITH THIS SERVER ANYMORE
Wrath of the Lich King seems to have a few servers left, however expect those to update to Cata when the time comes
Rising-gods.de
2500-3000 players
German, but support for English players
Everything scripted well, progressive content
Players focus on PvP, however the team is tends more towards PvE
completely blizzlike rates
Arena-Tournament.com
2500-3000 players
Intant level 80 and S7 equip
Farming better gear in BGs and Arena
Complete focus on PvP, all PvE is disabled
Most bugfree server out there
Pathfinding
Donation for rewards, nothing overpowered
Cataclysm has only 2 servers (that I know of) which could be considered as relatively bug free and big
WoW.freakz.ro
Up to 4000 players on 3 realms
Stuck on 4.0.6a for a while
XP rate x20, customizable by the user, blizzlike rates in endcontent
Most free of class bugs
Progressive content
Mostly Romanian but international support
Everything scripted
Focus on PvE
While being focused on PvE, this realm provides an insanely good amount of working spells and talents. I didn't think this was even possible for Cata yet, but they provide a really balanced PvP enviroment and I enjoy playing on there myself. The international realm lacks players, but be sure to check it out and be amazed yourself.
Monster-wow.com
Up to 5000 players on 3 realms
XP rate x12
4.0.6, updating to 4.2.2 slowly
Progressive content, therefore the update to 4.2.2 atm
Balanced focus on PvP and PvE
Completely international, subforums for every language
Please do NOT play on this server, unless you are really desperate for new content and you do not care about PvP or working spells/class balance in general