Hey, let's discount the ones you have to hunt for, and just grab the ones that fall right into your lap, and look at those.

Matra Magic is godly when you first get it. Cheap enough to spam, yet wipes out every group you face in one shot until at least the Corel Prison.

Flamethrower is pretty much useless, because, oh, hey, Matra Magic.

Laser is the first Gravity attack you get, and equal in power to Demi 2, and makes one of the best Morph aids in the game for picking up early Sources.

Trine may not be Beta or Aqualung, but it's still a solid addition to your lineup.

Frog Song, like most status effects, is usually more trouble than its worth, but it can come in handy in places.

Magic Breath is awesome. Sure, it's easier for enemies to resist than the other elemental nukes, but it does great damage.

Shadow Flare is powerful, even if it is rather pointless by the time you get it, since you have so many other hard hitting abilities by that point.