Ranking damage types is a fairly complex endeavor, but… I would rank them as following.
- Physical/Internal Trauma. Very strong throughout the game, not usually highly resisted, lots of resist-shred options. Good amount of skills that deal it or augment it (mostly in Soldier, unsurprisingly, but a little in Shaman, some in Demo, and some in Nightblade.) Lots of devotions that boost it well. Pretty solid support from itemization, particularly set items. Doesn’t rely on or need conversion at all.
Tie for 2. Pierce. Again, pretty strong throughout the game, not usually resisted in great amounts by most enemies AFAIK. Fair amount of skills that deal it or augment it, though these skills are mostly only found in 3 masteries (A little in Demo, a little in Soldier and a lot in Nightblade) at the moment. Decent resist-shred options, though not quite as many as physical/IT. Decent support from itemization, though I personally think it could use a little more. Relies heavily on conversion as no weapon in the game actually deals straight-up pierce damage to my knowledge, but conversion from phys to pierce is generally very plentiful via armor piercing on weapons and a certain constellation (though the Internal Trauma tied to Phys will NOT convert to anything Pierce-related).
Tie for 2. Bleed. Strong throughout the game EXCEPT when dealing with Undead-archetype mobs. Good selection of skills that deal it from multiple masteries. Good resist-shred options, arguably Bleed is nearly as well off as Phys/IT on this front. Okay support from itemization, could probably use more. Does not use conversion at all, as there are no ways to convert to Bleed. (I personally think Crate should condense Pierce and Bleed to share the same base damage type, one being the “standard” damage and the other being the “DoT” damage. Certain skills and devotions would need a slight rework, but I think this is entirely doable and even recommendable for Crate to consider for the future)
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Acid/Poison. Strong throughout the game with the exception of very particular mobs. Good resist-shred options. Decent selection of skills that deal it or boost it, though these skills are concentrated within only 2 of the 6 current masteries. Good support from itemization. Good-to-outright-fantastic support from devotions. Conversion doesn’t seem to be quite as plentiful for Acid/Poison as it is for some other damage types, but it’s still there in decent amounts.
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Lightning/Electrocute. Pretty strong throughout the game (the only enemies I know of that resist Lightning in great amounts are Mogdrogen, Valdaran and I believe the Gazer/Overseer enemies - Gazers are pushovers anyway). Decent resist-shred options. Skill support is… ok, as Shaman, to a lesser extent Demo and to an even lesser extent Arcanist all offer options for this damage type. Good devotion support, albeit not amazing. Fairly decent but not fantastic support from itemization (more is certainly welcome). Conversion is reasonably plentiful for Lightning/Electrocute.
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Fire/Burn. Pretty strong in most places, runs into mild-to-severe resist issues with certain mobs (it’s a pretty safe bet that if they look like they’re on fire, if they have “fiery” words in their name somewhere, or if they’re certain Chthonic mobs, they will resist fire at least somewhat). Decent resist-shred options, not quite as strong as Lightning but close for RR. Skill support is decent, probably slightly better than Lightning but not much. Great devotion support. Very good itemization support. Conversion is very plentiful.
Tie for 6. Vitality/Vitality Decay. Strong throughout MOST of the game, though it runs into issues with certain Chthonics who are outright immune to it without a resist shred. Okay resist shred options, but not great (chiefly because there are no devotion skill resist shreds that target Vitality resist in particular at the moment, meaning if you go Vit damage you had better have Shaman, Occultist or both in your class combo). Itemization support is a little lacking. Skill support is… ok. Devotion support is again, just ok (and not great). Conversion is not very plentiful.
Tie for 6. Chaos. Strong throughout most of the early to mid-game, though getting it at those points in time is not the easiest thing to do. Runs into a lot of enemies that resist it to varying degrees in later game (Chthonics and Bloodsworn). Very so-so for resist shred options. Skill support is somewhat limited, more so than Vitality. Devotion support actually seems better than Vitality. Itemization support seems better than it is for Vitality. Conversion is seemingly slightly more plentiful (could be wrong about this) than it is for Vitality, with one caveat: Chaos has no DoT component, so if you’re converting Physical to Chaos, any Internal Trauma damage you do will remain unconverted.
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Cold/Frostburn. Runs into resist issues throughout the game, save for perhaps the final act. Has alright-but-not-great resist shred options, and is notably inferior to the other 2 Elemental damage types here. Skill support is decent. Devotion support is fairly poor. Itemization support is alright. Conversion support is reasonably plentiful.
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Aether. Will encounter mild-to-moderate resist issues throughout the game (anywhere there are substantial amounts of Aetherials/Aether Corruptions), but is rarely resisted in any substantial amount outside of those enemy types. Has precious few resist shred options. Skill support is very limited, found pretty much only within the Arcanist mastery, but decent within that mastery. Devotion support is middling. Itemization support is so-so. Conversion support is somewhat limited and it runs into the same issue that Chaos does, as Aether also lacks a DoT component to convert to.
There are certain other “damage types” that I didn’t include in this list: Life Leech and Life Reduction, namely. I had no idea where to put them.
EDIT: I will say that life reduction is utterly useless against bosses. Life leech is, I believe, highly resisted by boss enemies, but they aren’t completely immune to it like they are with life reduction.