Since the most used classes are *probably* sinx and champ, most used are probably both poison and neutral; then perhaps fire and shadow. (I use fire personally since puppetring favors it). It's very rare for a sinx to use verters; at least from what I see.
Box of storms is a special case since you can see what they use; but I don't come across it that much lately.
If you're facing a gunslinger, expect holy. (Or neutral if they're smart enough to switch)
If you're facing a clown or sniper, expect ghost (just take off GR for that). If they're smart enough to switch, personally I'd assume either holy, shadow, fire or wind; in that order. (That's what I use, except that I favor shadow).
Be careful with shadow and holy though, if you immunize yourself to it they'll just swap and do double damage.
I use fire with puppetring, and wind with frost joker in some rare cases.
Occasionally I'll use wind or seismic verters; but I usually default fire.
One last thing to worry about is aspersio. If you're getting ganged by a whole guild (or two if you know who I mean), chances are they'll have a priest buffing them. If that priest is smart (fortunately all the smart ones either quit or aren't part of the ganging guilds); they'll aspersio their teammates to save them verters.
In that case; expect holy. Do note however that they could simply used cursed water to counter angeling and pretty much rape you. Then again if you really are getting ganged they'll have a champ or two so you'd need GR either way.
I don't really have a problem with elements; I only wear neutral and *sometimes* poison resists. (If I'm facing one of those stereotypical full buffed SB sinxs)
*Real* final last thing I forgot to mention: In WoE, expect a lot of sinxs to use cursed water; naturally.
Right. I realize this post isn't really addressing the topic issue but there's no way to know what verter people use unless you do a mass poll or something; so I won't answer for anybody else. I'm just giving some basic *guidelines* that I, based on my own personal usage, use to "guess" what element which class is using. Hope it helps.
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Edit: Final note;
If I'm on sinx and somebody's immune to poison; I don't even bother using a verter. I just dispell my endow and use neutral to hit them. I only use verters if they're obviously using devs etc.
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Edit 2: Okay, I lied. If I was on sinx and somebody was immune to poison I'd use sand attack to kill them.