It depends on what class/god combo and their mechanics on exactly what and how much you want to pick up. Some classes (dweomer-caster and elementalist) get a heap of free spell levels as they go up in level anyway, so you can pick up pretty much whatever you want. Worshippers of Azmodan also get free spell levels (especially priests), so you can afford to spread yourself around a bit, just boosting the spells you always use with Scrolls of Azmodan.
For everyone else, the advice in the previous post holds true. Between one and three main attacking spells, all of different elements, and a bunch of level 1 utility spells. Plus a summoning spell, although you don't necessarily have to level that one if you don't want to, it's handy at level 1 (especially empowered) just for the early meat-shields.
Most classes do get some free spell levels on their default attack spell (mostly up to level 10), which gives you reasonable chances of finding more power-ups for it no matter how jumbled your mix is. Less spells is better though, better chances to level what you want, plus you'll probably never use half of them anyway.
Best spells (IMHO):
Attack: Chain Lightning, Vaporize, Glass Spray, Ice Shards, Stoning, Shockwave.
Utility: Malediction, Haste, Healing Water/Remedy, Wind Walking, Invisibility.
Summoning: Conjure Elemental Servant (they're all good, but Fire is the handiest. More light and free Vaporize artillery), Summon Avatar of Darkness (yay regenning bone dragons), Summon Disciple of Light (healing, free crusades, teleporting unicorns).
The two best summons are actually Call Familiar and Call Hero, but they're pretty random on what you get. Familiars might seem hopeless at first, but no matter what you get, they're great. Give/buy them healing water with some spell regen for it and you'll always have a heal-bot on hand for when you need it (assuming you regen the familiar spell). Then dump all that crap that you'll never use onto them and be happy that it didn't all go to waste. I prefer my familiar or summons to do most of the heavy lifting in the healing/dispelling/enchanting department, so I can get on with killing stuff. Plus, sometimes the RNG gifts you something awesome as a familiar. Sometimes.
Oh, and try a Halfling Treasure-Hunter of Shaco some time. Every power-up is worth it's weight in gold. It makes the game feel very different (limited everything except run speed). Use haste, turn run on, and pray you find something good before your glass spray runs out. Lots of Fun in the new versions, and a fair bit easier than before. Shaco is better percentage-wise than Karma for item drops (200%x2 instead of 300%) and way better for spells, but Karma would probably give better items when they do drop.