Quoting myself because it’s quicker and easier (and darn it I’m kinda lazy like that)
Here’s the breakdown for Classic Era connected realms:
Anathema, Arcanite Reaper, Bigglesworth, Blaumeux, Fairbanks, Kurinnaxx, Rattlegore, Smolderweb, Thunderfury, and Whitemane [PVP West] 
Arugal, Felstriker, and Yojamba [OCE PVP] 
Ashkandi, Mankrik, Pagle, Westfall, and Windseeker [PVE East] 
Atiesh, Azuresong, Myzrael, and Old Blanchy [PVE West] 
Benediction, Faerlina, Heartseeker, Incendius, and Netherwind [PVP East] 
Earthfury, Herod, Kirtonos, Kromcrush, Skeram, Stalagg, Sulfuras, and Thalnos [PVP East] 
 
None of the roleplaying realms (PVE or PVP) are currently connected. Neither is the OCE PVE realm of Remulos.
According to reports, the high pop server cluster for PVE is the PVE East cluster of Ashkandi, Mankrik, Pagle, Westfall, and Windseeker (aka the Mankrik cluster) while the high pop for PVP is the PVP West cluster of Anathema, Arcanite Reaper, Bigglesworth, Blaumeux, Fairbanks, Kurinnaxx, Rattlegore, Smolderweb, Thunderfury, and Whitemane (aka the Whitemane cluster).
 
 
Welcome back, Maelenthor, and best of luck to you.
             
            
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