Okay Fire is my favorite but it isnt the best damage output and here is why,
Resistances for Aero and Hydro are almost always less than fire resistance so even thought the Meteorstrike says 475-675@ damage it will actually hit for much less since the resistance brings it down, also since fires status effect is burning, it may cause some damge if they do ignite, but it is usually short lived and low value comparitively.
Cold and Electricity are better since the target usually has less or even no resistance to them and being wet ( from rain or standing in water) increases damage and apparently likelihood of a status effect, elec usually stuns meaning a free turn or so to hurt them or whatever, cold spells can freeze also meaning them missing a turn. ALso Hydro and Aero have Tornado, Rain, Invisible, Featherfall, and healing spells, so very good to have. Not all spells are for combat useage.
Poison ( earth spells) do decent damage but I do not like the persistence and spreading uncontrollable nature of poison, it gets everywhere and spreads, damages whoever comes in contact with it and can be set on fire which is then another different hazard. You can be creative with poison, but I am reticent about its use.
That depends, no ? For 1 hero I put all ability points into Loremaster early on so with 2 rings (+1 to Loremaster) that hero had at level 3 already Loremaster 5 !
For example : The Undead ...
They have mostly a fire resist of -40, (crushing and/or piercing -50 )
so fire is then the best elemental spell. And a fire spell + midnight oil is a great combination

!
But I'm sure every kind of elemental damage can do good work if used wisely, depending resistences of enemies, the surroundings, the evolution the battle goes, the heroes current skills that aren't in cooldown time, etc ...

I play with Bairdotr & Wolgraff and I'm planning to give now (level 4/5) my ranger or rogue the Loremaster 5 cap (via 2 rings and an amulet = +3) because their initiative is higher ...
My 2 Source Hunters go for wizard spells : one with Earth and Fire, the other with Air and Water.
PLUS: one of the Source Hunters is also some kind of battlemage:with a mace, spear, sword or otherwise - to be short : depending the resist. for crushing, piercing or slashing he can change his melee damage
I love the four elementals and having them all four to my disposal gives me the most satisfaction playing D:OS !
No matter what's the most powerfull (elemental) damage output, such things do not interest me that much .
Just me