I'm curious what spells in GURPS Magic your players seem to favor excessively, or always put on the "must have" list.
Healing spells and Recover Energy are always taken if they are allowed. You simply are much less valuable without the latter, and healing is too valuable to be without in some fashion. In DF, healing spells (normally a wizardly prerogative in GURPS) are off-limits to non-clerics, so we don't see any on mages. But Recover Energy, always.
Great Haste. This is the number one spell I see taken. It's extremely valuable as a force-multiplier on your best fighters, and on the mage himself (who then casts twice as fast for a while.) It's so must-have that players occasionally feel limited in their magic choices - you can't make a mage without it, basically. Sometimes they do, briefly, and then end up leaning it. Few prereqs, extremely effective.
Teleport. If this wasn't allowed in DF, they'd have it there too. Cheap and easy and terribly effective. My last game was largely centered around teleportation and Gate magic.
Seek Earth. It's rare for my gamers to be without this spell. They figured out early on it was good for hunting treasure, and good for hunting bad guys with steel weapons, too.
Seeker. Another one they used so often I had to get very strict about it, and I still get a lot of vague seek requests or oddly phrased targeted requests. Still, it's how my players have determined where a lot of missing items have gotten off too. Great for "bring the widget to the quest-giver" scenarios for the players.
Levitation. Not as much anymore, since I nerfed it from its "cheap and easy win button" application, but for a long time Levitate was the primary combat spell in my games. Non-flying enemies without significant resistance were neutralized, just like that.
Shield. It's a rare fight without someone getting Shield +5 cast on them, or even higher thanks to higher Magery or Power Investiture. The massive jump in defenses allows for standing up to big crowds and holding your own against even offensively superior foes, and if they can't hit you they can't hurt you.
And I won't even start on Create Servant.
How about your guys? If you use or used GURPS Magic, what are the "can't live without" spells of your group? If you're not using GURPS Magic, feel free to comment, but I'm a wee bit less curious about spells I'm not using in my game . . .