I had to wait till I got to my books:
If Conjure Animals is 3rd level, and it can summon 1 beast of CR 2 or lower …
If Conjure Celestial is 7th level and can summon a celestial of CR 4 or lower, and at 9th level it can summon a celestial of CR 5 or lower …
If Conjure Minor Elemental is 4th level and can summon 1 elemental of CR 2 or lower …
If Conjure Woodland Beings is 4th level and can summon 1 fey of CR 2 or lower …
If Conjure Elemental is 5th level, and can summon 1 elemental of CR 5 or lower, but it goes hostile towards you if you lose concentration, and it can increase by 1 CR for every slot level higher than 5th …
If Conjure Fey is 6th level and can summon 1 fey of CR 6 or lower, but it goes hostile when concentration is broken, and it can increase by 1 CR for every slot level higher than 6th …
Conjure Animals > Conjure Celestial suggests a progression: 1st level is CR 1, 3rd level is CR 2, 5th level is CR 3, 7th level is CR 4, and 9th level is CR 5. But 1st level for CR 1 would be a bit much, I think; A CR 1 has a lot of power and is quite a bit more powerful than a 1st level character.
The hostile summons offer a 1 for 1 progression, but they come with a big risk. I wouldn't want 1st level characters to have that power.
My really back of the envelope assessment of a Greatsword Champion Fighter's CR was (if they spread out their abilities to last 2 fights per short rest and 6 fights per day):
Level) CR (prime caster spell level)
1) 1 (1)
2) 1
3) 1 (2)
4) 2
5) 3 (3)
6) 4
7) 4 (4)
8) 5
9) 6 (5)
10) 6
11) 7 (6)
12) 8
13) 8 (7)
14) 9
15) 10 (8)
16) 10
17) 11 (9)
18) 11
19) 12
20) 14
My point is that you wouldn't want a summon to be stronger than a player. How quick would this get out of hand?
Summon Monster
1st-level conjuration
Casting Time: 1 action
Range: 60 ft.
Components: V, S
Duration: Concentration, up to 1 hour
You summon a creature from another plane. Choose one creature of any type other than humanoid of challenge rating 1/4 or lower. The creature disappears when it drops to 0 hit points or when the spell ends.
The summoned creature is friendly to you and your companions. Roll initiative for the summoned creature, which has its own turns. It obeys any verbal commands that you issue to it (no action required by you). If you don't issue any commands to them, they defend themselves from hostile creatures, but otherwise take no actions.
The DM has the creature's statistics.
At Higher Levels: When you cast this spell using a spell slot of 2nd level or higher, the challenge rating increases by 1 step for each slot level above 5th. A 2nd level slot summons a CR 1/2. A 3rd level slot summons a CR 1. A 4th level slot summons a CR 2, and so forth.
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That would line up with Conjure Minor Elementals and Conjure Woodland Beings at a 4th level slot. Conjure Animals is still better at 3rd level. At a 7th level slot, it would be CR 5, which would be better than Conjure Celestial, so that sounds like a no go.
Welp, I took a stab at it. I'll go back to the drawing board.