I've noticed that there are a few skills that you absolutely need to take or otherwise offer so many benefits that you can't afford to pass them up. I've ordered these according to play style. Warrior (melee specialist) Dual wielding- maximum damage output means that you'll be getting hit less and therefore getting hurt less Wizard (spellcasting specialist) Blood Magic- gives you the spellcasting endurance that you need to take on monster zoos, especially potent with AOE attacks. Archaeology- gives +1 trap sight for free at level 1, "it belongs in a museum" will allow you to accelerate your ability to level up and attain the spells you need to take control of the game much earlier. "This translation is all wrong" will let you reroll your gear until it provides that much needed +1 mana regen per turn. wandcrafter (for the people that decide to do it) Piracy - allows you a constant source of gems, which you can use to make more wands Alchemy - gem transmutation gives you even more options for turning gems into wands, it also lets you make sulphur and other wand ingredients. with both of these skills together you should have enough materials to use wand attacks whenever you need them. Archer (crossbow specialist) Tinkering - Most important- allows you to craft lots of ammunition, upgrade your crossbow. dodging -or- ley walker -or- mathemagic -or- burglary- A source of teleportation/mobility is essential for fending off enemies with a crossbow.
Pretty much any build: Burglary. 1-2 level's worth of XP early - when XP is most important - with a single Skill Point investment (Lucky Pick). The amount of XP you get from Lockpicking Chests and Doors (and the easiest of traps, you get +1 , too) when you first start out cannot be understated. Mana-less Teleport (Move in a Mysterious Way) - requires a solid investment of Skill Points, but again - it's a mana-less teleport. One of the best "Get-Out-of-Dying FREE!" Cards in the entire game. It also allows you to access Artifacts and other items located on those normally-inaccessible islands. Stealing random items from Vending Machines (and later, Shops) can come in handy now and again. Not vital, but at least you're getting free items simply for taking the tree at all (Vending Machines), and the free Shop items (Five-finger Discount) are a nice little consolation prize for Skill Points spent as you work towards Move in a Mysterious Way. Lock-up is yet another way to reap the reward of massively powerful Evil Chest Weapons with a minimum of risk. Lock-up the baddie and pelt it with Bolts/Thrown from afar. Combine with Move in a Mysterious Way for extra ease of killing named monsters.
Gish: Fungal Arts (well, this is kind of for everything, but even better for a gish - mana, buffs, spell power, everything.) Necronomiconomics - high bases, good melee buffs. Viking Wizardry - melee buffs and high bases. Blood Magic. Fuels your buffs. -------------------------------Core Everything else is whatever you damn well please. I go Necrolyzing Aspirant from Faxpax, CK from CK, Burglary from core and thats it.