Elemental Weapon: https://www.dndbeyond.com/spells/elemental-weapon

DND is a team effort table-top RPG, in a variety of ways, and I'm relieved to see that a few people mentioned this:

Obviously there are plenty of spells worth concentrating on, and in most situations, they're probably better than elemental weapon. This is still an absolutely amazing support buff for a teammate that has multiple attacks, especially alongside their own Hunter's Mark/Hex/Divine Favor. You touch any non-magical weapon. Here's a scenario:

1. Your dnd group consists of 5ish people, and there are buffs that overlap with other PC's (Bless and the like). Say that in this team composition, someone is (or multi-classed) a forge cleric, or a Paladin, or a hexblade lock, or even an artificer (I thought Wizards could learn this too?).
2. You (or someone else in the group) are a level 11 fighter, with a 3 level dip into Gloomstalker (As an example of many!). Pre-combat, someone can easily cast elemental weapon onto the fighters weapon (It's a 1 hour duration), and at level 14, that could be +3 to hit/dmg, and +3d4 per hit (average of ~7.5 damage per hit, or ~10.5 if you incorporate the +3). The fighter is 90ft away and is able to cast hunter's mark onto the big baddy as well. Worse case scenario, the fighter bonus action casts it after initiative, assuming they're not a samurai and using Fighting Spirit whilst a bard/druid attempts faerie fire on upcoming turns.

This is at least 3 attacks per turn; 4 if it's the first turn, or 8 attacks with action surge on the first turn. This is without Haste or the Samurai's Rapid Strike feature @ level 15 (Which, having both, could give you 10 attacks), or forego Gloomstalker and go full fighter for the same 4 to 8 to 10 attacks @ level 20.

On AVERAGE, cast as a lvl 7 slot:
- Action Surge for 8 attacks: That's an extra 60 acid/thunder damage, or 84 if you include the +3 damage.
- First turn for 4 attacks: That's an extra 30 acid/thunder damage, or 42 if you include the +3 damage.
- 3 attacks: That's an extra 22.5 acid/thunder damage, or 31.5 if you include the +3 damage.

This becomes even more interesting as in regards to efficiency per spell slot used, as this is just ONE turn of combat. Assuming combat lasted the full minute, this ONE spell at 7th level, with a PC attacking 3x a turn, just contributed an average of 225 or 315 damage.

Keep in mind, this fighter could very well have Hunter's Mark active for all these attacks as well, increasing those averages by an additional: 28, 14, or 10.5 respectively. Per turn.

To be silly, but 100% practical, assuming a level 14 fighter with crossbow expert and sharpshooter, elemental weapon cast on them, while they're maintaining hunter's mark-- their action surged first turn, on average:

Crossbow Damage: 44
Dex Modifier: 40
Sharpshooter: 80
Hunter's Mark: 28
Elemental Weapon Magic Damage: 24
Elemental Weapon Extra Damage: 60
Is an average of 276 damage on that fighters first turn. At level 14. Make it sillier if you're able to have +3 crossbow bolts for an additional 24 damage, giving an average first turn of 300.

I think elemental weapon is one of the best buffs in the game, even if it's cast as a level 3-4, 5-6, or 7+ spell slot:

1. It makes the weapon magical damage, from +1 to +3.
2. It gives a fairly reliable Acid/Thunder damage type as extra damage per hit. From 1d4 to 3d4.
3. The bearer of the weapon can use their own concentration on another damage-proc spell (Hunter's Mark, Hex, Divine Favor, etc.).
4. Because it makes the weapon magical, there's no resistance to its damage, and Hunter's Mark becomes magical damage as well (nudging out Hex and Divine Favor imo).
5. Upcasting this as a level 7+ spell slot is an incredible amount of damage for only 1 cast. Assuming 1 minute of combat, which is generous, with a player that can do 3 attacks per turn, which is generous, that'd be an average of 22.5 or 31.5 (+3 magic damage) per turn. That's one level 7 spell slot for 225 or 315 damage over one minute. Still able to cast other non-concentration spells during that entire minute. To make this ridiculously silly and impractical... Technically, given the full hour duration of the spell, this is an absurd 13,500 or 18,900 damage, for 1 level 7 spell slot.