Add to that Shaman wins priest.

For hybrid, I wanna say it's Paladin. Ranger has a bunch of cool clickies, but don't benefit as much from general upgrades (which I believe is the spirit of the question) due to the AC nonsense and their spells being ass.

With SK, if you're a big boy, your stats are already pretty solid, so less benefit there, add to that you fear kite a lot, which means you're about as effective with or without big HP/AC items half the time. Bard obviously needs like nothing to be amazing.

Special shout out to Monk though. Really scales nicely with weapons, HP, and AC. Just doesn't need it as much as Warriors, which can do literally nothing without decent gear. There's definitely an interpretation of this question where Monk is the answer.

Epic turns the bard into a quasi skald. Singing Steel Helm lets the class to be in two places at once, fulfilling both the role of puller and support. Illusion masks allow for movement through doors that require a higher pick lock skill than the class is provided, making it more roguelike as a result.

Butcherblock Hammer turns the cleric into a battle cleric, and Blackstar to a lesser extent.

Oakwynd turns the ranger into an archer.

A Weighted Axe (incorrectly?) turns the warrior into an archer..

What else? Dawncall weapons start to transform the paladin into anti-undead, enough to where it encourages actively seeking out those targets.

If just gear is the consideration, Warriors and Monk scale the best.

Bards with the epic become more rounded and get some song upgrades (and basically another +Attack song).

A BFG ranger on the right target can rival geared rogues. Not on long fights or rooted targets but it basically adds 2-3 banes worth of extra damage in 3 mins.

If progression is level 60, all the classes become vastly better. Maybe the sham is most obvious since the whole play style changes with torp and malo.

+1 for warrior and monk. Monk avoidance/mitigation as you start increasing above 1200 ac.. man it feels so good. Even better as you go higher (and achieve insane HP pools).

Warrior is probably it. But saying Monk is out is kind of a crazy statement IMO. If we're talking 1-60 then they benefit hugely from gear upgrades. Maybe not as much in like the first 30 levels but from there definitely. There are for sure soloable XP mobs that Monks cannot handle in the higher levels without good gear and getting that gear broadly increases your XP options.

Putting Enchanters to the side since their gear upgrades are important if you plan to Charm kill. Wizards you also will need a high mana pool if you want to quad kite, so that's really impactful. Any caster will benefit from a GCD clicky. I think Mage benefits hugely from JBoots since you have no CC so in a lot of zones those JBoots can be the difference between life and death for you if things go south.

Kind of a nit, but quadding with jboots sucks. SoW pots are much better, so I don't see jboots as transformative. Flux staff, though.

I think they are transformative. It might suck with JBoots compared to doing it with SoW, but considering it allows you to quad kite vs not being able to quad kite I think that's transformative (and saves you from constantly having to raise plat to buy SoW pots)

I would have to say monk as IMO with full BIS everything they are the most powerful class in P99. Can DPS like a rogue, tank like a warrior, drop eyeballs like a mage, self heal like a pally, feign better than a necro.

I would say shaman is a close 2nd because torp and slow is wildly powerful, they get some pretty tanky gear at the top end, can add feign death, best debuff, root rot, and tons of mana free clickies (especially DPS)

In a straight fight, would a BIS fully itemized monk beat a BIS fully itemized warrior, face to face tanking/dpsing each other?

The monk would naked..

So if the warrior can beat the monk fully geared, then the warrior gets the most benefit.

Based on the last FFA PVP event ending with like 6 monks being the final 6 toons I think we can safely say monk is the best BIS class