ChromieCraft: the open-source server
What is the best World of Warcraft emulator? Let’s take a look to all available server application projects, for each different WoW client version.
World of Warcraft – Classic – 1.12.x 
CMaNGOS Classic
CMaNGOS has its own 1.12.x classic repository, based on MaNGOS Zero (a downgrade of the old MaNGOS 0.12.0 branch).
It’s playable, having a moderate development activity with most of the game contents available and working.
Official website: http://cmangos.net/
Repository: https://github.com/cmangos/mangos-classic
World of Warcraft -The Burning Crusade – 2.4.3 
CMaNGOS TBC
The 2.4.3 repository of CMaNGOS is based on the old MaNGOS 0.12.0 branch and it’s playable, with a moderate development activity. Most of the game contents are available and working.
Official website: http://cmangos.net/
Repository: https://github.com/cmangos/mangos-tbc
OregonCore
OregonCore supports the 2.4.3 game version of World of Warcraft and it’s based on TrinityCore. Its development is fairly active and it is playable.
Official website: https://oregon-core.net/
Repository: https://bitbucket.org/oregon/oregoncore
World of Warcraft – Wrath of The Lich King – 3.3.5a 
TrinityCore 3.3.5
Based on MaNGOS, it is probably the best wow emulator with very active development and having the largest wow emulation community. 3.3.5 has been the main branch of TrinityCore for years.
Most of the game contents are available and working blizzlike.
Official website: http://www.trinitycore.org
Repository: https://github.com/TrinityCore/TrinityCore/tree/3.3.5
CMaNGOS WOTLK
CMaNGOS has its own 3.3.5a repository but it is not as complete as the previous game versions, however it is quite playable.
Official website: http://cmangos.net/
Repository: https://github.com/cmangos/mangos-wotlk
NEW:Â SUNWELLCORE/AZEROTHCORE
Read this article to know more.
Official website: http://www.azerothcore.org/
Repository: https://github.com/azerothcore/azerothcore-wotlk
World of Warcraft – Cataclysm – 4.3.4 
TrinityCore 4.3.4
TrinityCore deleted the 4.3.4 branch but there are some developers who are still working on this game version. However it is not fully playable.
Repository: https://gitlab.com/trinitycore/TrinityCore_434
ArkCORE
It is based on TrinityCore 4.3.4, there is mainly one developer working on this project.
Official website: http://www.arkania.net
Repository: https://github.com/Arkania/ArkCORE-NG
CMaNGOS Cataclysm
CMaNGOS has one repository for 4.3.4 too but it is not playable yet and quite incomplete.
Official website: http://cmangos.net/
Repository: https://github.com/cmangos/mangos-cata
World of Warcraft – Mists of Pandaria – 5.4.8 
SkyFire
SkyFire is a 5.4.8 emulator based on TrinityCore. The development is a little active, the game quality can be improved.
Official website: http://www.projectskyfire.org/
Repository: https://github.com/ProjectSkyfire/SkyFire.548
World of Warcraft – Warlords of Draenor – 6.x 
TrinityCore 6.x
The 6.x is the main branch of TrinityCore since October 2014. The game is not fully playable yet, but the development is much active.
Official website: http://www.trinitycore.org
Repository: https://github.com/TrinityCore/TrinityCore
Summary
The Classic and TBC game versions haven’t many active developers but they are already quite complete, you can use CMaNGOS for both of them, or OregonCore (for TBC only) if you need something more TrinityCore-style.
The WOTLK game version is the most developed one, and TrinityCore 3.3.5 is a really good choice if you want to run a stable and playable server.
Cataclysm and MoP have never been properly developed and probably they will never be, because now the majority of developers is focused on the WoD game version using TrinityCore 6.x.