Eventually (depending on how quickly you level up), you will be able to select most if not all of the High Level Abilities. So the question is, which one do you want to get the earliest? It really depends on your playing style.
For example, if you like to hit your enemies with one big, powerful spell, Dragon's Breath is a great choice. If I recall correctly, it will kill a demilich instantly. I've used Comet before, but in most cases I'd rather use Dragon's Breath.
If you like to cast a bunch of spells as fast as you can in combat, Improved Alacrity is a great choice. If you use it in combination with Time Stop, you can do some pretty interesting things.
If you like to summon creatures to help you in combat, the planetars are about as good as you can get. You can summon more than one planetar at a time if you use a spell such as Project Image. It takes practice to coordinate your summoning spells and attacks (you don't want to cause harm to your own party), but it's a very effective playing style.
If you frequently run out of memorized spells, it's nice to pick the High Level Abilities that allow you to memorize extra spells. But personally, I'd rather have the chance to cast new spells.
I like to use Energy Blades in combination with Critical Strike when I play a multi-class character, and there are other nice ways of combining High Level Abilities when you play a multi-class character. If you're playing a single-class Mage, you don't have to worry about them, and you can concentrate on your Mage's strengths.
Naturally, all of the High Level Abilities are nice, so you need to focus on the ones you really like.
Good luck!