I recently replied to a similar thread asking about Classic Era realms. Below is a Copy/Paste of that response:
Classic Era are the realms you’re looking for. They are dedicated to not progressing down any of retail’s past expansions. Many of the Classic Era realms have been ‘connected’ (blizzard’s version of merging) according to timezone region/server type.
Additionally, blizzard has opened free transfers to Mankrik in the PvE East Cluster and Whitemane in the PvP West Cluster.
Those realms (and the ones they are connected to) are in all likelyhood the most populated for a given faction/server type. I play Horde on the PvP East Cluster holding together Benediction/Faerlina/Heartseeker/Incendius/Netherwind.
Good Luck and if you have any questions, ask away!