I don't know about GURPS 4th ed, being a 3rd ed player. But if things aren't that different, the obvious zap-boom spells are significantly different from games like D&D. GURPS magic tends to be much smaller scale, and with the need to satisfy prerquisites for spells, getting the good ones can often mean having a bunch of stuff you'll almost never use.

That said, Iron Arm is essential for melee combat, and Missile Shield is also a must have. Jet spells tend to be more effective than missile spells, while the Body Control college can really mess up opposition. And for maximum carnage, get Dehydrate up to 21+, to be able to hit enemies for 3d6 damage from a safe distance.

But never undersetimate the power of knowledge. I've had to retire wizards in GURPS games, simply because they were too good at gathering information. Sure, they mostly sucked in combat, but when you can cast a dozen different spells which let you find out what's going on, you tend to break the game pretty badly.

The thing to ask yourself as a caster in GURPS isn't "How do I put out a fire?" Rather, it's "How many ways are there to put out a fire and which ones do I want to be able to do?"