In other words: Which of templar, nightblade and sorcerer does the most PvE single-target damage?

Personally I'd avoid nightblade, because it's hard to do good damage with as a PvE DD.

Magicka Templar is naturally easy, but fun.

Magicka Sorcerer lends itself to so-called heavy attack builds. That's the easy play option for that class, though it only becomes available after levelling up and farming some specific gear.

Stamina Arcanist beats everything as a DD, if not outright then for ease of use.

The game is relatively balanced. The differences between classes mostly come down to ease of use. While the skill gap has shrunk, especially due to Arcanist, skill still matters. An experienced endgame player will easily do double or triple the damage of a beginner, not because they have access to better gear, but mainly because they know how to play and do maximum damage.

In the hands of a veteran endgame player, the classes are pretty close in terms of damage. In the hands of a beginner, templar and sorc will do more damage than nightblade, while (stamina) Arcanist beats everything.