An interesting list I found, top 15 MMA promotions:
https://sportytell.com/mma/best-mma-promotions/
1. UFC
2. Bellator
3. Absolute Championship Akhmat (ACA) / Previously ACB
4. AMC Fight Nights / Previously Former Nights Global
5. ONE
6. RIZIN
7. Invicta
8. Pancrase (Japan)
9. M-1 Global
10. Legacy Fighting Alliance (LFA)
11. World Fighting Championship Akhmat (WFCA)
12. Konfrontacja Sztuk Walki (KSW)
13. Deep International / Deep
14. Road Fighting Championship (Road FC)
15. Cage Warriors Fight Championship (CWFC)
Surprisingly no PFL / WSOF, they have Gaethje and Marlon Moraes
Top 20 but of all time:
https://www.mmamania.com/2012/3/18/2882556/ranking-the-mma-organizations-of-all-time