Spell Sequencer:

This allows you to store 3 spells of level 4 and lower, and enables you cast them instantaneously and without interruption. Some popular combinations include 3 Flame Arrows, 3 Lightning Bolts, etc... you can use it offensively or defensively, so just have fun experimenting with what you think works best for you.

Ruby Ray of Reversal:

This spell enables you remove Spell Traps, enough said.

Finger of Death:

One hit kills are always nice, and even if the target saves against spells, the spell still deals some damage. This spell works best in combination with Greater Malison.

Project Image:

Possibly the cheesiest spell for a Sorcerer. With this spell, you will not have a summon limit, you can cast every single spell you have times the number of Project Images you have plus one.

Mordenkainen's Sword:

This is one hardy summon and is quite difficult to kill by most means. The only setback is that it has a very short duration time. You only need it to distract your opponents while you cast your Time Stop though.

Khelben's Warding Whip:

You tend to settle most Mage vs Mage fights by using Time Stop, and Khelben's Warding Whip will not be necessary as you usually destroy the Mage the moment the Time Stop runs out.

Prismatic Spray:

Although it may seem nice, it is unpredictable and can destroy loot.

Sphere of Chaos:

Way too unpredictable, I prefer to have spells which I know what will happen.

Spell Turning:

Staff of Magi has Spell Trap already, and you can also have the Cloak of Mirroring.

Summon Hakeashar:

If I wanted to insult a Mage and kill him/her with a summon, I can always use a Skeleton Warrior.

Limited Wish:

I rather cast Limited Wish from a scroll, the benefits conferred by it are not really that great. Feel free to use it if you so desire, but make sure you have high WIS.