Check out this site. Warcraftrealms has a great huge list of all the realms, their time zone and their “type” (pvp, etc.), as well as a pretty good measurement of their population. For some reason, it refuses to list the realms in a descending order like they should be, but oh well. Scroll down to the bottom, that’s where you’ll see the realms with the highest Horde population. Make sure you glance over at the time zone and check whether or not it’s a PVP realm. I played on Rivendare and Arthas, way back when. They both seem to be a bit dead now.