Deleted my old post and trying again
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So obviously for a raid target the answer is warrior. For an xp group things are different.
What you have to understand is that the P1999 playerbase is insanely well geared compared to live. An untwinked melee at 55 is still going to have full planar, RBG at worst and probably a Cloak of Flames, epic if rogue or monk, sky gear, Kunark armor pieces etc. And half the melees are twinks and have even better gear than this.
Therefore generally mitigation isn't a problem. The primary issues are threat, dps, and xp penalty. Warriors clearly win at dps (although its not huge) and xp penalty, so the question is whether the (essentially perfect) aggro of a paladin or SK combined with their utility (snaring runners, extra CC, etc) compensates. And this basically depends on the warrior's gear. Because warriors got screwed epic-wise compared to rogues and monks. It's very hard for a warrior dual wielding 9/20+14/28 with 130 dex to keep aggro off a Rogue with 15/25+10/18 or a monk with 9/16+17/28. In that case I think Paladins or SKs are probably better. On the other hand, if the warrior has epic/vp weapons then I think warriors are probably better.
TL;DR: Warriors are the most gear dependent class in everquest.