I don't know if I am a good Templar tank or not, yet I can survive in dungeons well and I can survive for some time, sometimes more, when the healer dies. My only trouble is taunting a big group of minions, yet I'm looking for a way to improve that. I have no trouble with bosses, except when there is adds or when the healer dies.

First, an excellent skill is sun shield and it's morphs. It gives you a good shield and it damages everyone around you, helping with aggro.
For aggro I have ramstack(don't know how to spell), which is the morph of puncture, the first 1h+shield skill. It taunts an enemy for 15 sec, decreases their armor and it increases yours. The increase of your own is weak, yet it's better than nothing.
Some people say that focus rune is perfect for a templar tank, yet I don't like the fact that you have to stay on the area. I know that you keep the bonus after you leave for 8 secs, yet I prefer immovable(heavy armor skill). You can't use both, they don't stack, so you have to choose. I prefer immovable because of the mobility, the fact that it uses stamina, and because I want the set master armorer. Yet I don't know which is best. I may be wrong.
Another one I use is bone shield, yet I want to change it. It stays for a short time, and I don't have enough stamina for it after I use immovable and ramstack. You may want to use it, because it gives a 30% health shield, and damage to melee attackers (depending on the morph).
My last skill is honor the dead (first restoring wrath ability). It is instant, so it is good for emergencies, and it gives magicka recovery for a time if you are below 75% health. I don't like the other morph, because the heal is divided for more players, and this skill is just for myself.
My ultimate is remembrance (ult for restoring wrath). It is great for the "OH F***" moments. If you are almost dying and there is no healer this skill will save you. I have lost count of how much times this ult saved me. If there is a better tank ultimate please let me know, because this one is awesome.
In my other bar I have dps skills, so that don't count for tanking. Basically you can have whatever you want on the other bar. I like to have ranged spells and AoE, but that is your choice.
I use 1h+shield and Heavy armor.
The passives you can take are all of the heavy armor ones, most 1h+shield (or all), depending on your class you can take those passives, some aedric spear, maybe undaunted (I did), and the others you choose for yourself.
I hope this was helpful for you.

Yes, sorry I wrote it from my memory.

I'm interested in magicka tank build to, I've got a l20 templar that's playing bank at the moment