The ranking of the top 10 fastest-growing online casinos may vary over time due to market trends and new entrants. However, as of now, some of the leading and rapidly growing online casinos include:
1. Bet365 Casino
2. 888 Casino
3. LeoVegas Casino
4. Casumo Casino
5. PlayOJO Casino
6. Rizk Casino
7. Mr Green Casino
8. Dunder Casino
9. Genesis Casino
10. Royal Panda Casino
Please note that this list is not exhaustive and subject to change. Additionally, growth rates can vary for various reasons such as geographical location, marketing strategies, and player preference.