Historically, Improved/Greater Invisibility and Fly were bread and butter for arcane spellcasters. With the new concentration rules, not so much, although I recently added a spell to my game that lasts 1 minute and gives the benefit of both spells at 5th level (concentration).
Phantasmal Force is great to lure an enemy into a trap. I like to use it to get an enemy in position and then use another spell, such as Mold Earth, to trap them. I recently used used Phantasmal Force to lure an enemy into a 30' deep, 5' wide pit. Then I used Mold Earth to bury him under a lot of dirt.
Tongues and Mass Suggestion.
Minor Illusion (for sound) and Silent Image.
Delayed Blast Fireball and Fireball for the really big boom.
For Bladesingers: Contingency with Dimension Door and Haste - when Haste ends, teleport to the back ranks while you recover.
Magic Mouth and Silent Image. Cast Magic Mouth on something and have it trigger when it is thrown. Then, when you cast silent image, toss the something to have the image speak your prepared words, effectively.
My favorite combo is generally not legal, but is so crazy - an archer with both Haste and Swift Quiver. I had a high level EK fighter/ H ranger / A rogue fire 10 arrows in a round and deal over 200 hundred damage - he had a quiver that cast swift quiver requiring no concentration once per day.
