Appendix 1: Spells
For the following I have considered spells from the Basic book of B/X D&D. Other editions and other games will have different spells, but hopefully the reasoning carries.
Level One
Sleep: Bar none, the best spell in the game. The ability to reliably take out a whole roomful of 1HD enemies makes Sleep indispensable for parties up to mid levels, and single-handedly makes the Wizard a viable character at first level. Opinions vary on whether slitting the throats of your sleeping victims is strictly Lawful, but even absent that it remains the number one first level spell by a long margin.
Charm Person: Dungeon expeditions succeed or fail on the strength of their intelligence gathering. Charm Person creates a willing, eager informant out of any intelligent dungeon denizen. Even a short-duration charm is more than enough time to get useful information, and the fact that they might feel a little violated afterwards is little downside at all.
Level Two
Invisibility: What makes Invisibility so powerful is its duration. It lasts until the target attacks or casts a spell. For a scout, neither of those are part of their regular duties. That means for the price of a Level Two spell you get the services of an invisible scout for the whole day. Over a few days you can make the whole party invisible. The major weakness of Invisibility is how it treats light sources - invisible scouts must scout in the dark, or else carry a light which gives away their presence.
Continual Light:This spell solves lighting problems for the party forever. The huge strength of this spell is that, being permanent, it can be cast in downtime, providing the party with near-infinite light sources. Casting Continual Light on arrows or crossbow bolts provides the party with a way of projecting light to counter enemies lurking out of sight. The ability to cover and then suddenly reveal a magical light source gives the party advantages in both stealth and in surprise, dismaying light-sensitive enemies.
A perennial threat to low-level parties is being attacked with ranged weapons from outside the range of their lighting. Continual Light and other light spells create possibilities for countering this strategy. Cast on the end of an arrow or bolt, you can project light at a distance.
Level Three
Fireball: The game changer. There’s not much in the game that’s not intimidated by 5d6 damage. Even if they make the saving throw, Fireball is instant death for most 2-3HD foes. Once the Wizard learns Fireball, any Fighters in the party are relegated to second rank.
Cleric spells
The Cleric is often relegated to the role of mobile Cure Light Wounds platform. I think this is a waste of the Cleric’s potential. Cure Light Wounds is of course extremely useful, indispensable even in campaigns with standard (1HP per day) healing. But that’s a downtime concern. Prep and cast Cure Light Wounds on your day off. In the dungeon, there are better uses of your precious spells. Cleric spells tend to be rather situational, so it’s hard to choose the overall best spells. But if I had to choose, at first level Protection from Evil would be my pick. Cast on the front-line Fighter or the scout, it provides all-day benefits, and the complete immunity from many enemies can be invaluable. The other option is Light, which has great practical utility, and doubles as an offensive spell.
At the second level it gets even harder. Find Traps is frequently useful and Hold Person is also a strong choice, especially if you’re expecting powerful humanoid enemies.