You already have a DK tank you don't play. So what's wrong with that one? DK has good group utility and a get-out-of-jail free card that makes it easy to learn, e.g. Magma Shell. That skill is also the single most meta option to go super far in the Infinite Archive, if you ever wanted to do that.

Nightblade is possibly the easiest to play, due to the combination of Dark Cloak while standing still with Siphoning Attacks. The cost of the latter is easily outhealed, while the stamina (and magicka) return while blocking arguably borders on broken. So does Magma Shell, but for sheer ease of use nightblade may just beat everything right now. Is it the best for hard modes? I'm not sure about that.

Sorc has a skill that gives you +40% block mitigation. This is very strong and sorc has quite good group utility via the atro ultimate. That said, sorc's stamina sustain tool, Dark Deal, is harder to use than nightblade's (Siphoning Attacks) or DKs (Earthen Heart skills), because it drops block. You will invariably die as a newly-minted sorc tank from dark dealing at the wrong time, but then again learning not to perma-block is something you may want to do sooner or later on any class.

Arcanist also has a skill to increase block mitigation, albeit only by up to 20%, and some other unique options, such as an armor-stealing taunt. It also majors on shields, possibly like sorc or better than sorc in that regard. I believe it's a strong choice, but I haven't played it as a tank.

Templar is not usually recommended, because it lacks in group utility as a tank and it lacks a decent health-scaling heal. I believe people don't rate the templar shield skill. It would be fine for just vet trials, just not special in any way.

I see necro tanks reasonably often. Can't say anything about them, nor about warden, since I don't play those classes as a tank or at all.

I'll just add that I like the DK chains a lot. They allow you to taunt unmovable targets for free, e.g. bosses or adds that have already been moved and are, therefore, CC immune. I should say I'm a vet trial player and have only done a little of the harder stuff, such as vCR+2 and vAS+2 in terms of hard modes. I play mainly with PUGs, not organised groups. That, and natural inclination, means I like to keep things easy for myself. DK chains are something you can freely spam without running down your magicka much and without a risk of taking away taunt from something the other tank has hard-taunted.

Boss fights are often well-defined, but it's the trash pulls in vLC, vRG, vSE, and that very last one in vDSR, which I loathe a little. My coordination is not the best, so I run in, hard-taunt maybe two of my designated targets from range with the ice staff taunt as I'm running, then switch to DK chains and grab as many things as I can without the risk of taking stuff away from the other tank. Pull skills operate like a 2nd class taunt. They only taunt if the target isn't already "hard-taunted" with a proper taunt (by the other tank).

Up until scribing, the only pull skill available to most classes was Silver Leash. That drains your stamina like crazy at the most dangerous moment in trash pulls: The beginning. Now that scribing is available, I would recommend you create yourself a magicka pull skill. Unfortunately, if you also play your sorc in PvP, there's a strong case for turning Wield Soul into a PvP spammable instead.

In terms of sets, the one I am married to in virtually all builds is the Master's 1H+S on the front bar. Magicka recovery jewelry enchants are bread and butter for a tank, but Indeko cost reduction enchants are particularly efficient. I typically use 1x of those and my Puncture hovers around a 1K stamina cost. With or without such an enchant, that skill is typically cheap enough to make for a very useful heal, despite it costing stamina. Master's 1H+S also synergises with Tremorscale. Don't use it to reach resistance cap (too unreliable), but using it to go over the cap boosts the penetration that Tremorscale gives.