20 Best World of Warcraft(WoW) Addons
Z-Perl is more an alternative to the standard UI see below whereas ElvUI is more like a full rework of the entire WoW design.
Overwolf will almost certainly require accounts, at least for paid subscriptions and Sync.
However, if you're like me and accumulate a large number of addons, you'll want a standalone addon manager.
This addon scans your immediate map and shows you the rare mobs that are near you.