Levels 10-14: Improved Fireball the best dps increase you can get at this level, since your Rank 3 Fireball has a base 2.5s casting time, this lets you get them off at 2.0s each.

Levels 15-16: Flame Throwing is nice because range is convenient and lets you get in more fireballs when fighting mobs. Also, Ignite can be put off since your Intellect (and thus crit rate) is still pathetically low.

Levels 17-21: Ignite is a raw damage increase for when you crit.

Levels 22-26: Improved Arcane Missiles because it's time to build Arcane to get Instant Arcane Explosion for use in instances.

Levels 27-31: Arcane Concentration to get clearcasts.

Level 32: Evocation for mana regen once every 10 minutes... at this level it's going to be around 100% of your mana pool, which is very nice. Be warned, it starts fading away from there around level 40ish and at level 60 you can expect about 50% of your mana pool back from Evoc.

Level 33-37: Improved Arcane Explosion is very good for the instances you'll be doing at around this level and up -- Gnomeregan, Razorfen Downs, Uldaman, Temple and on up really appreciate it.

Level 38-39: Incinerate to make Scorch more useful in Instances, since you'll now be able to scorch for a clearcast to fire Arcane Missiles on.

Level 40: Pyroblast because it's the best place to drop a point in Fire and you might as well get it. It's really not that great... though it is slightly more mana efficent than Fireball but I haven't done tests to see whether that justifes the extra three seconds it takes to cast it as an opener vs. a mob.

Level 41-45: Improved Scorch because Scorch is very much an PvE Instance spell and this talent makes it all that much more mana efficient. Impact won't really do that much for you in instances and Blast Wave isn't worth the points in Flamestrike for its PvE utility.

Level 46-48: Critical Mass for more critical strikes... damage increase.