If people talk about an expansion pvp wise they usually talk about the last/best season (which usually is the same), S8 for WOLTK, S15 for MOP etc. And lets not forget we are on a private server, forever stuck in the last season of each patch. If i tell you that MOP was amazing for pvp i mean season 15, not the beginning.
Not to mention that i havnt even talked about warrior being the most defensive spec, just that a hybrid of prot holy in relentless gear in 3.3.5a wouldnt last at all against a BiS geared Arms warrior. Arms war is the strongest spec in 3.3.5a woltk with full pve gear for the only really active bracket, 2v2 (yes its not the best 1v1 class, far from the worst though, warriors have won 1v1 tournaments, yes they also werent as OP in 3s cause of the Wizards).
And aksing for the most OP defensive spec in the game is a pointless question anyways without any specifications. Is in in 1v1s? Or 2v2s? Or 3v3s? World pvp? Bgs? Wod turbo comes to mind or lsd/p in any expansion as super defensive minded comps in 3v3s. Double healer arms war to stay at the theme of arms war during late woltk. Double blood DK sometimes during mop before that got nerfed to hell. Sl/sl lock, demo locks in late vanilla. A rogue could go to 0 vs anything if you just played lame enough.
Without dampening certain healers in wod were unkillable 1v1. Since you can kill any spec in a 1v1 in Woltk (i think) it by defaul beats everything here? Its not a question you can answer imo.