My game, level 16 now.
Paladin spells are used for smite.
Wizard spells:
1st slots are mostly dedicated to shield, although I have seen high level magic missiles used it hard fights to finish off enemies, or simply do reliable damage.
Most of the other levels have spells that are useful cast at their level even at high levels.
I've seen magic missile boosted, hold person, banishment, fireball (doesnt scale great though so not often), and scorching ray. They're the common ones.
Cleric spells.
A lot of the staples here are designed to be boosted. I quite often use boosted healing spells, boosted spiritual weapon, hold person, banishment, etc.
Other spells:
I see Moonbeam boosted all the time. It scales really well. I've boosted disintegrate against held creatures before as that does some really nice damage, usually up to 8th level, as there aren't a ton of really good 8th level spells.
I also see rituals used a lot still at higher levels. Detect magic, alarm, silence, and so forth.
Oh and also dispel magic and counterspell. Always being boosted especially by my NPC spellcasters.
The bigger issue IMO is known in my game as "Dave's School of Drow Wizardry", I.e. In combat very few spells are actually useful. Combat is over so fast, there is not much in the way of pre-buffing, and concentration mechanics limits what NPC spellcasters can really do in 3 rounds.
The result of this is all casters end up with the same spells, shield, counterspell, greater invisibility, misrt step, balh blah blah.
I've tried building a variety of different casters, necromancers, illusionists, poison based Drow casters, etc, and they basically have all sucked so bad in combat, they weren't worth the time spent creating them.