Oh, you mean useful skills. I maxed Lockpick (you can get 2 levels in Verdistis), but that was more functional than favourite. Restoration is handy to help conserve potions early in the game (you can get 3 levels from quests), but you'll eventually be able to just rely on
restoration potions.
Assassin's Kiss is ok if you want to stick to a specific character type, but even with it maxed you are not going to do as much damage as with a sword. The game is also fairly biased towards swords, with more potentially good swords consistently in the game than other weapon types (IIRC there is on 1 dagger always in the game).
Magic Barrier is good to learn once you have some spare skill points you don't know what to do with, though eventually you will get most of your resistances from equipment bonuses, etc. Augment Damage and Augment Defence should be maxed for a mostly melee character (a level of one of them can be gotten in the Dark Forest).
Meteorstrike is good, since can be cast quite fast and has a bit of a stun effect. Hell Spikes is quite effective once you get it to level 4 or 5 (one of the toughest bosses in the game uses Blind + Hell Spikes). Deathly Disks is good against moderately close range opponents.
The survivor's Curse spell can be cast repeatedly on a target to dramatically lower its resistances (it can not be resisted, so is much better than the mage's Withering Curse spell), and can then be followed by poison or elemental damage, or a skill like Polymorph.
Telekinesis is handy; I maxed it with purchased skill books (
Free skills, and spell books you can buy).
I think I was around level 20, or so, before I entered the Ars Magicana sewers.