This thread has information on the Classic Era realm clusters:
The tl;dr would be that the NA PvE East cluster containing Mankrik and the NA PvP West cluster containing Whitemane would have the largest populations since they are the ones set up to receive free transfers.
There are smaller communities outside of these two clusters and all of the NA Classic Era clusters share the same battlegroup for BGs so I would encourage you to roll a lv 1 on any prospective realm and try chatting with the locals to find the place that’s right for you.