Here are tested units sorted from best to worst:
1. To hold 18 with pure butchers I needed 3060 value (7 butchers + zombie), 2870 value leaked 5 units. Absolutely incredible performance comparable with ea on lvl 13.
----Edit 8/19/2020: in 3.44i 2460 value (6 butchers) holds (2 rows by 3 or oracle build), adds like witchdoc, parasite or troll makes them super OP.
2. Pure doppelganger with 3640 value (8 of them) leaked 1 creep (very luck dependent unit tho).
3. 3870 value of master of fire (upped druid, 9 of them) held.
4. With trident I needed 3870 Value (9 of them), very luck dependent unit and considering the perfect armor/att composition, its result is kinda disappointing.
5. To hold with LoD, I needed 3900 (13 of them). LoD > hades for this level.
6. 3980 value of cavaliers (9 cava + a knight) was needed, kinda luck dependent units also.
7. To hold 18 with pure blasters I needed 4000 value (10 blasters).
8. 3965 value of ToT (13 upped + 1 small) leaked 2 units.
9. 4000 value of wolves (16 of them) barely held. The most luck dependent unit of them all
10. 3900 value of warchiefs leaked 8 units, however 4200 held with huge mid. Let's say you need 4150 here
Test with thralls went in a similar way, but in normal conditions I'd go for thrall rather for tanking abilities and dmg behind.11. Pure GoD with 4160 value (13 GoDs) leaked 3 creeps.
12. To hold 18 with pure spawns I needed 4240 value (16 spawns).
13. With ancient wandigoo 4300 value was needed (10 units).
14. Neotanks needed 4320 value. Neos > doomsday on this level.
15. To hold 18 with pure oceanus I needed 4500 (!!!) value(almost full 3 rows). Aqua/oceanus absolutely blows lvl 18.
16. Krogoths needed 4610 value (9 of them + a cyborg).
Keep in mind I ran all the tests only once so aggro and spawn luck are a big factor there, that doesn't change the fact that aqua is awful and butcher absolutely stunning tho
Testing conditions were 1v1-like, therefore in 4v4 these values needed should be even less and I can totally see 2870 butcher guy in mid 
