Mythic Odysseys of Theros added two more CR 4 celestials: the winged bull and winged lion.  Both are good at charging enemies, and the lion can even knock them prone, which is fun and useful.

Spoilers for Tomb of Annihilation:

For the right campaign, there are more CR 5 celestials in Boo's Astral Menagerie: a starlight apparition and a mercane, which have their uses.

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Thanks to Planescape, this spell now has a lot more targets:

bariaur wanderer, hound archon, lantern archon, musteval guardinal, sunfly

The dabus is only an option if you're in Sigil.

I'd say the hound archon is the best one out of these overall.

The most powerful part of this spell is the shape change ability. It can polymorph into any CR4 humanoid caster. It can refresh "spell slots" of the humanoid caster by returning to its true form and polymorphing into another creature just as an action. Some CR4 humanoids can cast hold monster, cone of cold, plane shift, cloud kill, arcane eye, telekinesis etc. Except you're a cleric, but you now have access to warlock, sorcerer and wizard spells. 

https://docs.google.com/spreadsheets/u/0/d/1HFliCW365VtgMbYtAFzXmklrf7aDo2F35EyquecgPmg/htmlview#gid=729155693

In order to make this spell actually viable:

1. Summon a Couatl if used at 7th lvl, or Battleforce Angel/Felidar at 9th lvl. For this setup, I'd recommend casting it into an inverted Magic Circle, although you can just ask it to enter afterward to increase the chance of settling this diplomatically
2. Ask the Celestial nicely if it would like to serve you for the next 180 days
3. If the answer is yes, skip step 4. If no, proceed down the list as normal
4. Use the inverted Magic Circle you set up beforehand to entrap the creature. Using Hallow and various Spell Glyphs to ensure that it cannot escape is essential. Tasking another Spellcaster with casting Silvery Barbs and/or Private Sanctum is recommended
5. Once preparations have been put in place, cast Planar Binding. I'm pretty sure that RAW, if the Celestial consents, it can choose to fail the Saving Throw intentionally. If it doesn't consent, that's what Silvery Barbs is for. Fair warning: most Celestials have Magic Resistance and very high mental scores, which makes it quite the pain to force them
6. Once the Celestial has been successfully Bound, you now have a servant with more versatility than the Wizard's, as long as you used the 9th lvl version because they can just copy it with Wish otherwise

This spell is really bad they is not much celestials to summon