I truly mean no offense, but that's a very vague question. There's a fair amount of things to consider before you can fully answer that question.
First, what do you intend to do? Are we talking PVP? PVE? PUG finder groups? Premade optimized groups? End game content? Hate meta? OK with meta?
For PVE for example:
DK tanks are generally considered 'best' based on the DK class having a very strong tanking toolkit and passives. Wardens are very good, Nightblade tanks are also viable but usually found in more optimized end game groups.
That said, all the other classes are capable to get the job done, again it comes down more to what content you are going to run and WHO you are going to run with. If you are in an end game group, this may very well be dictated to you.
Again, kind of the same with healers. Templar healers are very strong healers as they have a very good healing toolkit. Warden healers are also very good. Like I said above though, any class can do the job.
I don't main a tank, but one of my core progression tanks (mains a DK) has a Necro tank alt that he 'messes around on'. His Necro tank has done fine in base game 4 man dungeons, the few DLC dungeons he's run it in and vet craglorn trials. However, when it comes to DLC trials, he only runs his DK.
A Necro tank can do the job, it's just that a DK can do it 'easier'. It's just a different play style though really.
Now, like I said before, it also depends on who you run with. Some end game groups may very well tell you DK, Warden, or NB tank or go home. Keep in mind it's their group and their rules, but that's not an indicator that a necro tank is bad, it just means they don't want one.
Short answer is if you really like tanking you'll probably get more than one. If I had to recommend a first one: DK, Warden, or Necro as they are easy to learn and include the most built in utility (the classes were designed to tank from the ground up). Nord is excellent for both non-max CP and end-game meta.
As an independent reference a team of 2 has gone ahead and completed most DLC dungeon HM's and vBRP (just two people). Depending on dungeon they used Sorc, Dk, or Warden.
For the current trial meta of stam stacking NB/DK are mostly preferred. Warden, old main, used to be in there but I rarely find a use for it in trials because the almost ever present warden healer covers everything they do. Necro is a meta trial option too (see all mag godslayer). Note though trial meta has more to do with what tank classes augment the rest of the group optimally rather than the optimal tank class itself. IE if templar DD's and healer were nerfed into the ground you'd hear everyone suddenly saying Templars are the best tanks.
Dungeon Healers:
I'm assuming primarily healing rather than hybrid...
IMO the following three things are important for difficult dungeon healing
Minor Vulnerability: NB best, Warden good, Necro has it
Unique non-ultimate Synergies: Necro 2, Temp/Ward/Sorc 1
Honorable mention to best group dungeon heal though: Sorc
Just wanted some thoughts or what are strong for both
Thank you
There are actually quite a few good options for tanking. The best remains Dragon Knight in my opinion, due to its skill set. But Warden, Sorcerer, Nightblade, Necromancer... these can make very good tank builds that are perfectly viable for vet/trial content, and fun. My favourite tank is my Warden Nord tank, for instance.
Healing, not so much diversity. Templar remains unequivocally the best option to heal. However you can make viable healers from several other classes, most notably Warden and Necromancer, both of which I have taken into vet content with no issues.