im assuming your also talking about the hard modes as well?

just regular vets i dont think are really too difficult overall, as mechanically its all the same as normal with just more dmg from everything, but its the hard modes where things get tricky as they add/change some of the mechs

Never tried any of them on Vet, I just wait for the helmets to come to the Golden Vendor. ZOS has made the game soft.

With the newer DLC dungeons, regular vets are pretty accessible and I don't think it's worth talking about them. For example, while mask-farming Coral, we'd just do regular vet, 11 minutes per run (that's the entire dungeon) and we just ignore all the gryphons and burn the boss. It's quite a far cry from HM.

That is to say, I'm assuming that we're only talking about HM.

Broadly speaking, there are two types of difficulty.

• Difficulty of execution. Make one misstep and you die. Gotta keep your head on a swivel, have good awareness, and good reaction times. Fang Lair, Castle Thorn, and Red Petal Bastion are examples of this. These types of dungeons usually aren't hard to figure out at first, but they can be hard to no-death and their difficulty don't fade quite as much as you gain experience with the dungeons.
• Difficulty of strategy. These are fights where the difficulty comes from figuring out how the mechanics work and then devising a strategy to deal with them. Maarselok, Graven Deep, Moongrave Fane, are examples of this. These fights tend to be a lot more difficult initially when you're figuring things out, but once you've gotten the strategy down, they become much, much easier.

So, for example, we cleared Red Petal Bastion HM on the first pull on first day of PTS. We had a lot of deaths, but the fight is pretty self-explanatory. So, initial difficulty wasn't that high. Half a year later, when it came time to farm the motifs, we got only two or three trifecta runs out of the many dozens of motif runs that we did (we're not specifically going for the trifecta at this point, so not resetting and these are just the ones that we happened to get while motif farming). Now, contrast that with Graven Deep. We were not able to clear the HM on the first day of PTS--we wiped on it so many times that we ran out of time and had to try again the next day. But when it came time for the motif farm half a year later, we got so many trifectas during the motif farm that I lost count. This is a good illustration of the difference between difficulty of execution versus difficulty of strategy. Graven Deep's initial difficulty was much, much higher, but that difficulty fell dramatically once we developed a strategy and had practice with it. Whereas RPB was never about strategy and was always about being able to dance around on a floor filled with AoEs, and while this kind of difficulty isn't that high initially, it also doesn't abate as much, which is why at the half-a-year-later mark, I'd consider RPB to be tougher than GD, even though GD stumped us very hard at the beginning.

In no particular order...

• Coral Aerie: This is probably the hardest, because the tether mechanic amplifies the penalty of making a mistake, since if a tethered person dies, they're taking someone else with them. And if the tank is tethered, and their tether partner makes a mistake and dies, it will likely result in a wipe. The other difficult thing is positioning. The checkerboard waves, area of denial AoEs, and tether will often force people apart, yet there are also strong DoTs that need to be healed, and dispersal makes this difficult. I've gotten the Coral trifecta with several different groups, and the group that took the longest to get it (multiple weeks) was a group of melee DDs without good self-healing, and the group that took the least time (we got it on our second run) had ranged magicka DDs with shields and self-heals, because they can handle the "mechanics forced us apart and we just got the DoT on us" problem.
• Graven Deep: This is a classic all-the-difficulty-is-in-the-strategy fight. The final boss was tough at first, but once we developed a fool-proof strategy, it became very easy. How easy? I took some friends who had never done the dungeon and we cleared HM on the second pull deathless. And you don't need very high DPS. So it's hard for groups that don't have a plan and are just trying to brute-force their way through it. But it is downright easy if you have a good strategy.
• Stone Garden: Sadly, this dungeon's final HM was gutted right before the dungeon's release. It had such fantastic mechanics, like the kind of tank swap that you see in vHoF triplets or vDSR twins. But ZOS ruined the dungeon by removing CC immunity from the husks, which makes possible the current strategy of just stacking-and-burning them. Instead, all we have now is that annoying health-scaled DoT on the tank. It's still hard because of that DoT, but that's just a can-you-keep-enough-heals on the tank difficulty, not the kind of engaging mechanics difficulty that we had when the dungeon was on PTS. The original Stone Garden HM from the first week of its PTS is probably the hardest and best-designed dungeon HM ever. But now it's boring and nothing more than a heal check. Humbug.
• Lair of Maarselok: It's one of my favorite HMs. One of my fondest dungeon memories was doing this boss's HM on the first day of PTS when nobody knew how the seed mechanic worked. We banged our heads against the wall trying to figure it out, and then that Eureka moment when we finally pieced together how the mechanic was supposed to work was so euphoric. This HM is hard only at first, when people are getting used to doing the HM seed mechanic. Once you've nailed that down, though, it's pretty straightforward and not that difficult. I do like the teamwork and communication aspect of it.
• Frostvault: Once you have a strategy for doing the maze (we go as a group of 4, clear two pylons, return to mid to recharge, and then split into two groups of 2 to clear the final two pylons), it's pretty straightforward. I do like how distinct the roles are in the HM maze mechanic and how it requires good teamwork.
• Bal Sunnar: First boss is pretty tough for a first boss. Areas of denial everywhere, yet there is a strong heal check that the AoE-denials make difficult to heal. Second boss is pretty chaotic and stressful on a tank, esp. if the tank is also interrupting (when I tank this, I pop a siege shield for the range extension so that I can interrupt from the middle of the room). Final boss's heal check is stressful for the healer.
• Shipwright's Regret: Definitely the hardest first boss HM, mostly due to the double-bomb mechanic at the end. I would argue that the other two bosses are easier than the first.
• Earthen Root Enclave: The mechanics are pretty simple and straightforward, but there is just so much incoming damage on the first and last bosses.

Honorable mentions...

• The Dread Cellar: I don't consider this dungeon hard because the optional guardians are so overpowered. Their buffs are strong (particularly the fire one), and they act as res sigils when activated.
• Red Petal Bastion: While the amount of incoming damage on the last boss makes this a difficult HM to no-death, it's very recoverable as there are no mechanics that domino deaths into a wipe. I wouldn't consider RPB's HM to be difficult to clear, even though it's pretty difficult to no-death.
• Scalecaller Peak: Increased damage levels has made this dungeon much easier than before; with enough damage to push the boss quickly from phase to phase, you will see significantly fewer cone mechanics, which are the biggest killers.