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.
Daylight: Situational, especially in an edition where 2/3 of the races have darkvision. Don’t waste your limited list of spells known on this when you can get light or continual flame.
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: Technically very situational, but the power it gives you against enemy enchanters is unparalleled.
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.
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.
Protection from Energy: a really great preventative buff. The downfall is that it requires concentration, so it’s designed to fall eventually.
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.
Stinking Cloud: Sleet storm has a better area and roughly similar effect, taking creatures out of combat for a round or two.
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.
Tongues: If you’re the party spokesperson (with your high Charisma) you’ll want this so you can communicate with any creature.
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.
Water Walk: Flight is better, but until you have a way to give everyone flight this will do. However, the long duration and lack of concentration make it a decent all-day buff.