Just looked at the offensive spells in the spell list, and here are some observations:
Fire has Fireball and Fire Bomb, which are about the same, but Fireball is lower level and cheaper, and therefore superior. (If you plan on using lots of fire magic, make sure to pick a fire spell early so you can start gaining spell points sooner.) Psionic Fire, learned by the Psionic, is actually the strongest 2nd level attack spell; in other words, Psionics are better offensively than Mages early on (Psionic Fire is 1-8, Magic Missile only 1-7).
Water has Iceball, which is the strongest attack spell that targets a group. Mage spellbook only, but it means that anyone who wants to focus on offensive magic will want to take a water spell (like Frost) early.
Air spells do less damage, but there are some that cause Nausea while doing damage, which might come in handy. Stink Bomb hits only 3 enemies, but it does as much damage as Noxious Fumes (which is higher level and more expensive). Shrill Sound is useful early for its ability to hit an entire group at power 1, but at high levels you don't need that. Deadly Poison is an interesting spell that sadly didn't make it into Wizardry 8.
From Earth, Blades > Whipping Rocks, unless your level is low and hitting the entire group is more important than raw damage. That means that Priests are better off than Mages in terms of offensive damage from this realm.
Only Psionics get mental attack spells, but notably Psionic Blast causes both damage and insanity at the same time to potentially an entire group.
From Divine, two things of note: 1. Make Wounds is the strongest 1st level attack spell, which might not be what you'd expect. 2. Magic Missile isn't as good as Psionic Fire, and it is never capable of hitting the entire group (at power 7 it only hits 8 enemies). On the other hand, if you can find decent rechargeable spellcasting items, Recharge Item (available to both Mages and Priests at 90 Thaumaturgy/Theology) could be useful if you save first.
In conclusion:
Mages get the best raw damage, particularly from Fire and Water.
Priests get the best Earth attack spell, surprisingly, as do Psionics.
Alchemists are strong in Air, though their most damaging spell (level <7) is a Fire spell.
Psionics get a good 2nd level Fire spell, and their strongest attack spell is Blades, but they get the nice Psionic Blast spell (damage + insanity to group).
Note that this analysis excludes things like resistance, or the power of cloud spells which might be useful in longer combats, or instant death spells (are they useful in Wizardry 7?).