DAoC had the best PVP of any MMO hands down.
Built in 3 realm/faction system, made for constant action and the best dungeons in the game we're open to all contested which made for some great memories, also a honorable mention was warhammer online. Links provided below to servers that have actual population.
DAoC Link
https://playphoenix.online/
Warhammer Link
https://www.returnofreckoning.com/about.php