Did not vote because at least two class is needed for majority of the groups.
I believe Nightblade and Warden combo is most safest for raids. Which is most preferred for me.
Some groups drop Nightblade and use Arcanist for shields and buffs that they can provide, but it is not worth in some cases in my opinion because in some raids majority of the damage taken is oblivion damage which goes through shields.
For other contents it is rather same but full scale groups in Cyrodiil probably runs five or four healers which have different classes and provides different things to the group. I suspect a Sorcerer healer or support for negates and easy Minor Recovery bonuses through Regenerative Ward skill. High health Arcanist for Gibbering Shield and Chakram Shields. Maybe Necromancer healer for synergies since class has access to two strong synergy. Nightblade healer can also run ultimate regen sets to spam barriers but not sure how it will be effective with Battle Spirit modifiers. I would prefer all of these classes present in a full scale group because of these reasons. Once all of these things are combined you are basically immortal.
Theoretically yes. But practically a good squad composition can give a few % advantage, which can be crucial for vet stuff.
Building on this, some classes can also give certain buffs or bonuses that others just can't. Wardens in particular give Minor Toughness by default, raising max HP by 10%, and while that's not super useful to a DPS, it can be a lifesaver for the tank. It's actually part of the reason why wardens are generally considered the best healers.
I play the healer in my group, so the choice for me was easy - templar. But I find healing on a templar to be easy mode vs. healing on other classes. It could just be an L2P issue, but I find wardens to have more directional heals which makes positioning more important, and so your groups need to be more highly coordinated in terms of positioning for it to really work out IMO.