Never be afraid your question will be dumb, we love talking about Crawl, it's why the thread is 1000 pages
While Aoi is absolutely right when it comes to mid and late-game (basically 1-5 runes), even easier spells can severely limit the danger of most situations in extended (5-15 runes). Note that if you're planning on casting spells, you probably don't need the 30+ strength I have; you would want to get the minimum strength to cover the armour you're wearing (11 in the case of my Ice Dragon Scales) and dump the rest in Int. Also I'll need to first get a staff of Wizardry, but there's a ton of unID-d staves around in Elf.
The (majority of, since I'm in a hurry and can't really remember all) spells I'd consider on a general non-Trog melee champion are, in order of ease to cast:
- Apportation - always good to be able to pick up items in two turns, without physically dragging your carcass over. Incredibly more useful for Chei worshippers, since they'd have to spend 50-60 turns dragging
- Blink - allows you to escape being surrounded; it always has the potential of landing you in greater trouble, but if you're thinking about abandoning Trog this should be your first priority.
- Summon Butterflies - allows you to break Line of Sight; I never remember to use this spell, but I keep learning it on nearly every single character
- Regen - depending on your god, this is an absolute no-brainer for the Hells
- Lesser Beckoning - your very own "Get over here!"; used to be great everywhere and amazing in Tomb, nowadays just great everywhere
- Song of Slaying - even extended often has chaff you can clean up in one or two hits; this allows you a nice cushion of +Slay
- Passage of Golubria - the poor man's controlled blink; the better players I watch use it over and over and over, I myself am not really in the habit, but it's a great spell nevertheless
- Portal Projectile - another no-brainer if you're using ranged combat; allows you to safely dispose of casters and tormentors in the back lines
- Spectral Weapon - best when used with a Polearm, since you can double-hit a single enemy in a corridor, but great with any weapon at all, since at least it will damage and occasionally distract that Pan Lord you're trying to kill before he mutates you further
- Deflect Missiles - a powerful defensive spell, offset by the difficulty to cast it
- Silence - disables Liches and Mummy casters, but demons aren't really affected; if you can cast it a good spell but not something I actively work towards
- Controlled Blink - your most powerful escape option
- Tornado - not really feasible, but oh man
On my particular MiFi I'm going for three select spells:
- Lesser Beckoning
- Spectral Weapon
- Controlled Blink