Animate Dead: A solid summon, but amazing for a necromancer. Undead don’t tire, don’t need air/food, and aren’t likely to have feelings, so when you need something disposable and don’t want to offend anyone who cares about animals, these are your workhorse summons. The duration is long, allowing you to get a lot done with them.
Bestow Curse: This is the pinnacle of debuff technology, giving you options depending on who you’re up against and exactly how you want to hinder them.
Blink: A decent buff that isn’t as useful as it is convenient in that it doesn’t require concentration.
CatnapXGtE: An interested spell for quickly short-resting. If you’re a cross-class Sorcerer/Warlock you can chain this for great effect.
Clairvoyance: A great spy camera with a huge range and decent enough duration.
Counterspell: An obvious choice for caster duels. Negate their spells 1-for-1.
Dispel Magic: The only thing better than magic is the ability to get rid of it when it’s causing you trouble. This is a must for any party.
Enemies AboundXGtE: When fighting large beasts that try to stomp on you, look for the one with the worst intelligence save and let him do the heavy lifting for you.
Erupting EarthXGtE: less damage than fireball, but it also creates an area of difficult terrain, which keeps enemies grouped closer together, which keeps them in range for fireball.
Fear: a non-violent way to deal with groups of enemies.
Fireball: This is the yardstick against which all other AoE spells are measured. It does great damage in a huge area with long range. Whenever you choose a spell to know think to yourself “is it better than fireball?” If not, just upcast fireball.
Flame ArrowsXGtE: Not worth the spell slot. Just cast fireball and hit all the damage at once.
Fly: Flight is the best mobility option available.
Gaseous Form: Situational, but a great way to infiltrate, scout, or escape.
Haste: The best buff around. Any character benefits greatly for this, specifically one with heavy attacks. .
Hypnotic Pattern: one of the few effects that can remove creatures from combat and doesn’t give them additional opportunities to save to end the effect on their turn. This spell is incredibly powerful at low levels, and remains relevant at later levels.
Intellect FortressTCoE: This is your best protection against spellcasters or enemies who rely on saving throws to stun you or otherwise trip you p.
Leomund’s Tiny Hut: Eight hours of blissful slumber waits you inside this five-star resort of a dungeon shelter. With this spell, you can complete a multi-day dungeon crawl without needing to come up for air.
Life TransferenceXGtE: This is “healing” but at a higher cost than you’re likely going to want to pay. Wizards already have few enough hit points, they can’t really be giving them away, especially not at the cost of a spell slot.
Lightning Bolt: Line effects are really hard to use. You can reliably hit two enemies, but getting more than that is complicated. Great damage on this though.
Magic Circle: This spell is very situational, but it is essential to any sort of occult/supernatural campaign that will see you summoning or binding outsiders.
Major Image: The best illusion spell. Period. You can do almost anything with it.
Melf’s Minute Meteors: Complicated. It consumes a lot of actions to use properly and completely, but deals a lot of low-damage bursts at several targets.
Nondetection: This is a very good spell, but you’re not going to need it most times. You can usually get by with pass without trace from arranger or druid, and some stealthy characters like a rogue and monk. Stacking this on top of invisibility is overkill.
Phantom Steed: A budget travel option. If you can buy./rent/build/steal a wagon, your magic can ritual summon ahorse and you can sit in the back recasting it once an hour to keep it going.
Protection from Energy: a really great preventative buff. The downfall is that it requires concentration, so it’s designed to fall eventually.
Remove Curse: Situational, irreplaceable, the Cleric’s job, and also available to prepare tomorrow. Put it in your spellbook, but only prepare it when you need it.
Sending: This is very useful, but you won’t always need it. This is a good spell to get imprinted on an item, like sending stones instead of worrying about spell slots for it.
Sleet Storm: It doesn’t deal damage, but in the case of trying to take someone alive that’s a good thing. Also, if you’ve got support in the form of other powerful combatants merely hampering your enemies is enough to let them get into position.
Slow: A really great debuff that severely handicaps the target. Be wary, it’s considered a charm effect, and some enemies have immunity to them.
Speak with Dead: This is either mandatory for plot reasons, or can be replaced by a history check. However, it’s the best way to extract information from an enemy who is particularly dangerous. Kill them first, ask questions later.
Spirit ShroudTCoE: Bladesingers can get some good use out of this, though there are probably better spells overall to use.
Stinking Cloud: Sleet storm has a better area and roughly similar effect, taking creatures out of combat for a round or two.
Summon Lesser DemonsXGtE: This is utter chaos. You don’t get to control the demons, they just….do their thing.
Summon FeyTCoE: Better than demons, not as good as undead.
Summon ShadowspawnTCoE (Optional): Slightly better than Fey, but still not close to undead.
Summon UndeadTCoE: Choice of zombies or ghosts gives you excellent utility either in combat or for scouting missions.
Thunder Step: It takes an action to cast. Misty step is more useful for mobility, and any other AoE is better for damage. In the rare case you need both, this is your go-to.
Tidal WaveXGtE: Being knocked prone isn’t too much of an inconvenience in general.
Tiny ServantXGtE: This is basically a sentient sentry. It can set a watch and do simple tasks, but is still limited. That said, the fact that you can remain alert while asleep or otherwise preoccupied while this is active makes it extremely useful.
Tongues: If you’re the party spokesperson (with your high Charisma) you’ll want this so you can communicate with any creature.
Vampiric Touch: A very efficient use of a spell, in that it gives you several rounds of options. However, it’s purely a melee attack option, so it’s dangerous to use for wizards.
Wall of SandEEPC / XGtE: It’s going to slow enemies down for a single round. For that short duration, you’ve got control spells at lower levels that might work out for you.
Wall of WaterXGtE: Warding Wind is generally better and a lower level.
Water Breathing: Situational, but the long duration and lack of concentration makes it a solid all-day buff.