Blindness / Deafness was a surprising upcast choice
We discovered, during play, as my bard used this spell to debuff various enemies, that blinding a lot of enemies by upcasting blindness on them (with a high enough spell save DC, it was striking how often the save was missed) really made the battlefield change. I upcast to levels 4, 5 and 6 on a number of occasions in Tier 3 play. A couple of great aspects to the spell are that: (1) it does not require concentration and (2) if the enemy can't see the party, there are a whole host of spells and abilities that cannot be used until the save is finally made.
That it gives allies advantage when attacking the blinded opponent was a key reason I'd use it, since I was playing the bard as a support caster.
I saw similar results with blindness as a DM when the cleric player used it as a debuff.
While it was occasionally useful at lower levels, blindness became a lot more useful as my bard's Spell Save DC went up and she could impact more enemies. Unlike hypnotic pattern, it didn't have the "charmed condition" rider that makes that nice debuff not useful versus creatures who are immune to the charmed condition. (As CR goes up that number seems to increase).
- @Darth Pseudonym, in a comment, made a point that is worth repeating:
Blindness is particularly powerful against spellcasters, who tend to (1) have poor Constitution saves and (2) lose the use of many spells because when they can't see the target of a spell (to include counterspell) they can't cast it.
Two other spells that I see upcast with some frequency are fly and invisibility. Upcasting provides the benefit to more creatures, which can tactically have a substantial impact, and both of them can also have an impact outside of combat. (10 minutes of flying gets you over a non-trivial number of obstacles, the whole party being invisible for an hour allows for quite a bit of movement, infiltration, evasion, etc in many cases).