The Classic project has been divided into two launchers: Progression (Currently in TBC soon to become WotLK) and Classic Era (pre-TBC; will NOT progress into TBC/WotLK). Classic Era has then been divided into Realm Types of Seasonal vs. Non-Seasonal.
A poster has already given you the reference realms for the Progression servers.
For Classic Era Seasonal (Season of Mastery, which is currently in phase 5/6 and slated to conclude late fall-early winter of 2022) the PvP realm of choice is Jom Gabbar and PvE is Shadowstrike.
For Classic Era Non-Seasonal (fully phase 6 progressed, all raid encounters unlocked) there are Connected Realms arranged into clusters by time-zone (Note: This is NOT Cross-Realm. The connected realms are effectively merged). Here is a snip of the Bluepost detailing all Classic Era Clusters:
Furthermore, the NA West Coast Clusters have been assigned a destination realm to receive free transfers from all other other applicable Classic Era Realms: PvP is Whitemane and PvE is Mankrik hence, these two realms will be the most populated. I play on the NA East PvP Cluster holding together the realms Benediction, Faerlina, Heartseeker, Incendius, and Netherwind. If you have any questions, ask away.