Some quick thoughts:
-Summon Greater Demon seems like the most appealing, but I play in a party that gets easily annoyed by things like Wild Magic or summoning demons that could turn on us
-Tanarukk (CR 5, VGtM) - hits hard and has -1 to its CHA save is nice, but Magic Resistance is a problem
-Barlgura (CR 5, MM) - probably the best all-around choice, reckless and three attacks means you can summon it to the enemy's back lines and have it wreak havoc with spellcasters who might be relying on concentration checks. -1 CHA means you have a better chance of hanging onto it.
-Babau (CR 4, VGtM) - some potential utility with its at will spells and weakening gaze, has +1 CHA
-Chasme and Vrock (CR6, MM) - both have magic resistance, but have the potential to temporarily lock down an enemy horde's front line with Drone and Stunning Screech respectively. Seems like there'd be better uses for a 5th level spell slot.
