This may not be particularly useful information, but I found in interesting if nothing else.
I've spent the last few hours observing the amount of damage each magic attack will do against the same group of enemies--those enemies not having any inherent strengths/weaknesses against a particular type of magic.
Here are the results, from highest to lowest. To simplify things, the rankings of each are weighted, from lowest (1) to highest (76). The spell with the rating of 76 causes approximately 76x as much damage as the one with a rating of 1.
This is against ONE ENEMY with no weaknesses/strengths against magic, and with ALL STATS for ALL CHARACTERS maxxed out (magic **, power **, and HP 1 where applicable).
DoublevBomb (Lucca+Robo): 76
Grand Dream (Marle+Frog+Robo; Gold Rock): 48
Frog Flare (Frog+Lucca): 34
Final Kick (Crono+Marle+Ayla): 29
Gatling Kick (Crono+Lucca+Ayla): 29
Fire Tackle (Lucca+Robo): 27
Ice Tackle (Marle+Robo): 26
Blaze Kick (Lucca+Ayla): 26
Cube Toss (Marle+Ayla): 22
Twister (Crono+Robo+Ayla): 18
Arc Impulse (Crono+Marle+Frog): 18
Ice Toss (Marle+Ayla): 17
Fire Punch (Lucca+Robo): 16
OmegaFlare (Magus+Lucca+Robo; Blue Rock): 16
Super Volt (Crono+Robo): 14
DarkEternal (Magus+Marle+Lucca; Black Rock): 13
Fire Sword 2 (Crono+Lucca): 13
Ice Sword 2 (Crono+Marle): 13
Fire Whirl (Crono+Lucca): 13
Flame Kick (Lucca+Ayla): 11
Antipode 3 (Marle+Lucca): 11
Spire (Crono+Frog): 11
Red Pin (Lucca+Frog): 10
Fire Sword (Crono+Lucca): 10
Ice Sword (Crono+Marle): 10
SwordStream (Crono+Frog): 9
Shock (Robo): 9
Delta Storm (Crono+Lucca+Frog): 8
Delta Force (Crono+Marle+Lucca): 8
Fire Zone (Crono+Lucca+Robo): 8
Luminaire (Crono): 8
Area Bomb (Robo): 7
Glacier (Marle+Frog): 7
Antipode 2 (Marle+Lucca): 7
Flare (Lucca): 7
PoyozoDance (Marle+Lucca+Ayla): 6
Tail Spin (Ayla): 6
DarkMatter (Magus): 6
Line Bomb (Lucca+Frog): 5
Mega Bomb (Lucca): 5
Fire Whirl (Crono+Lucca): 5
Dark Bomb (Magus): 4
Dark Mist (Magus): 4
Max Cyclone (Crono+Robo): 4
Rocket Roll (Crono+Robo): 4
Ice Water (Marle+Frog): 3
Antipode (Marle+Lucca): 3
Ice 2 (Magus): 3
Lightning2 (Magus): 3
Fire 2 (Magus): 3
Laser Spin (Robo): 2
Fire 2 (Lucca): 2
Water 2 (Frog): 2
Lightning 2 (Crono): 2
Ice 2 (Marle): 2
Lightning (Crono): 2
Napalm (Lucca): 2
Water (Frog): 2
Ice (Marle): 2
Fire (Lucca): 2
Slash (Crono): 2
Flame Toss (Lucca): 1
(By the way, these were tested against Spekkio's Nu form. To find out about how much HP damage each attack did, multiply their rating by 177. That is, except for a few such as DoublevBomb, which I had to extrapolate from other battles because they couldn't hit Spekkio.)
All techs that aren't listed are either physical attacks or healing/defense, so they have a rating of 0 for this. Those techs are:
3D Attack, Aura Beam, Aura Whirl, Beast Toss, Black Hole, Blade Toss, Boogie, Bubble Hit, Bubble Snap, Cat Attack, Charm, Confuse, Cure, Cure 2, Cure Beam, Cure Touch, Cure Wave, Cyclone, Dino Tail, Double Cure, Drill Kick, Drop Kick, Falcon Hit, Frog Squash, Haste, Heal, Heal Beam, Hypno Wave, Kiss, Leap Slash, Life, Life 2, Life Line, Magic Wall, Protect, Provoke, Robo Tackle, Rock Throw, RocketPunch, Rollo Kick, Slurp, Slurp Cut, Slurp Kiss, Spin Kick, Spin Strike, Spincut, Triple Kick, Triple Raid, Twin Charm, Uzi Punch, Volt Bite, and X Strike.
Whew, that's a lot of info...