Shadow priests are pretty badass even with dungeon gear when it comes to 1v1. Rogues, like others have pointed out, get to choose their fights so you kind of have an unfair advantage passing those you won't win against and getting the drop on those you choose to engage.
I'm personally going to be going warlock and they're probably the best 1vMany. You can find videos of vanilla locks taking on 5 man groups out in the world. There's a high skill cap but if you can get there, you can stomp just about anyone 1v1.
-ClassicRaids
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