Here I will outline the best races in my opinion for stamina based Dragon Knight tanks. What I write here is an advice. In the end any race can tank, but some races just bring additional benefits to the table.
Argonians have strong passives in terms of resource management. They will get extra resources back when they use potions. This makes them a solid choice for tanking. But there is more. The argonian has also got a bonus to healing received and healing done. Increasing the heals you get, but also the vigors you cast. Next to that it will also get some max magicka and max health. All in all a very solid race choice.
The passives of the imperial increase both your max health and your max stamina. This means that you will get more stamina just by unlocking the passives, but it also means that because of the extra health passives you can spec even more attribute points into stamina. These passives make resource management a whole lot easier on an imperial, plus you can pretty easily switch to a DD build on the go.
3. Orc / Nord
The Orc and the Nord are both ‘ok’ races to tank with. Both of them get increased health which is nice, so we can spec more attributes into stamina. The choice is now between the extra healing received from Orc or the extra damage mitigation from Nord. Note though that damage mitigation might sound really ‘tank-like’, but it is only nice on paper. In practice this passive is barely noticable and most likely will not make a difference at all.
In terms of pure off-tanking the Argonian is the undisputed champion (not that I say ‘pure off-tanking’ as their status as a damage dealer race is meh). Their passives in resource management while drinking potions combined with their extra healing passives makes them a very flexible support player. Off-tanking is mostly based around vigor, and with an Argonian you will definitely get the most out of that skill.
Imperial is another strong choice as an off-tank. Their increased stat pools will scale the effectiveness of your vigor heals quite a bit. Another reason why Imperial is a good choice is that an off-tank will need to switch between damage dealer and off-tank during certain trials. Imperials make fine tanks and fine damage dealers. Giving them the advantage of flexibility.
Khajiit have two passives that contribute to off-tanking. First their stamina regeneration. As an off-tank you are by far not blocking all the time, meaning you will benefit from your stamina regeneration passives quite a bit. Next to that the Khajiit have a passive that increases their weapon critical. Vigor crits based on your weapon critical rating. More crits equals bigger heals. Next to that Khajiit are one of the, if not the strongest race as stamina damage dealers. So they are flexible as well.