Magic Missile:

I do not really need to explain why people should choose this spell really... it is already common knowledge that this is the spell for any Mage or Sorcerer. Each missile deals 1d4+1 magic damage. You get 4 missiles at character level 7, and 5 missiles at character level 9. 5 missiles do an average damage of 15 damage. Not bad for a level one spell.

Spook:

Spook may seem to be an insignificant spell, but there is a nifty thing about this spell. It has a saving throw penalty based on the caster's level. At character level 2, the target must save versus spells with a -1 penalty. At character level 4, the penalty increases to -2. This goes on until it reaches a maximum penalty of -6.

Friends:

A disclaimer, this spell is only useful in the beginning. I say only the beginning because that is when you have limited resources( read, money). After a few quests, you will be drowning in money whether you like it or not since you are soloing, plus the fact that most of the best equipment for a Sorcerer are gotten free. This is still handy though, if you want to buy the Robe of Vecna and the Bracers of Defence 3 cheaper. A solo Sorcerer will only buy these items, aside from various scrolls like, Freedom and Protection from Undead/Magic.

Identify:

Most Mages choose to stuff their level 1 spell slots with nothing but Magic Missiles. As a Sorcerer however, you get to have the luxury of having other level 1 spells. Identify is a very useful and money saving spell. If at any time, your Thief/Mage/Bard fails to identify an item, or you run out of scrolls, then your Sorcerer can help out.

Chromatic Orb:

When your Sorcerer reaches level 12, this spell becomes a weaker version of Finger of Death, it also deals acid damage, which you can use to kill Trolls if you ever run out of other Troll killing spells.

Blind:

Useful at first, it becomes resisted by everybody towards the end. Not a spell for long-term usage.

Protection from Evil:

You will eventually get the Staff of Magi, which grants this protection when equipped.

Burning Hands:

The damage it deals may be decent, but it also means being close to the enemy. Not a very ideal situation.