Hmmm...I'm soloing a sorcerer now, but I've played one through with a party before. Here's some of my picks:
Level 1
- Magic Missile: one of the most useful spells in the game.
- Chromatic Orb: the level 12+ orb is a poor man's finger of death (for a level one spell)
- Spook: the -6 to saving throws really helps
The rest you can choose on your own. Identify is pretty good, Shield can help protect against Magic Missile, and Friends helps with buying stuff cheaper. Not much else for this level.
Level 2
- Mirror Image: awesome defensive spell, pretty much a must-have
- Glitterdust: an area effect blindness that also reveals invisible; pretty useful spell
- Melf's Acid Arrow: a damaging spell that spreads out its damage, helping to disrupt spellcastin
I also usually take either Web or Horror (depends on my party and/or my mood at the time), and also Vocalize.
Level 3
- Melf's Minute Meteors: solid spell, especially against creatures like golems
- Skull Trap: this spell can hurt if you don't use it right (not much range), but it really hurts the enemy
- Flame Arrow: a very strong attack spell (this is a very good level for offense).
I usually like Dispel Magic and Haste at this level as well.
Level 4
- Greater Malison: a pretty much must-have; later opponents will make all of their saves without this spell
- Stoneskin: this is a must-have (not even pretty much must-have...definitely must-have).
- Improved Invisibility: useful for sneaking around party members, and the bonus to AC can help in combat
I usually also take Emotion (Confused opponents can still attack) and Spider Spawn (especially if I took Web at level 2; this combo can take you through a good part of SoA).
Level 5
- Breach: another must-have. Only sure-fire way to take down Stoneskin, PfMW, etc.
- Animate Dead: take once you get to level 15 (Skeleton Warriors rock)
- Lower Resistance: if you plan to do more than just breach down defenses for your tanks, you'll need this spell later in the game
- Spell Immunity: an overlooked spell by some people, this spell will save your bacon more than once.
I usually round out my list with Feeblemind (person just stands there and drools while you cut them down).
Level 6
- Chain Lightning: party friendly attack spell; great in closed quarters
- Death Spell: you'd be surprised at some of the Hit Dice of some of the creatures in the game; this will take out a lot of creatures, and all summons.
- Improved Haste: turns your tanks into chainsaws, without fatiguing them.
- True Sight: the best illusion-killer out there.
I usually finish it off with Mislead or Protection from Magic Weapons (usually the latter).
Level 7
- Project Image: greatly increases your spellcasting ability (use the image's spells instead of yours); wait until you get chain contingency
- Mordenkainen's Sword: great summon. period.
- Ruby Ray of Reversal: an Alteration spell that takes down magic defenses (thus, gets around Spell Immunity: Abjuration). Affects any level protection, as well.
- Spell Sequencer: extremely useful.
I usually round out with Finger of Death.
Level 8
- Abi-Dalzim's Horrid Wilting: your best area attack spell (except if you have ToB... ).
- Protection from Energy: great protection for all of the energy damage you will face along the way.
- Pierce Shield: knocks down a spell protection and lowers magic resistance all at once.
- Spell Trigger: very useful.
I usually round out with Symbol, Stun
Level 9
- Time Stop: do I really need to explain this one?
- Chain Contingency: just like the sequencer and trigger, extremely useful
- Shapechange: great spell, especially after you cast Time Stop...
- If you have ToB, then Wish (really don't need to explain)