There are many existing sports gambling sites nowadays and choosing the best out of them is really hard to tell. Thats why the answers would depend on the experience of the gambler itself.

Nitrogensports is the first betting site I used to bet but that changed as time passes and because they had an issues with their players which I dont want to experience myself. I explore to other sites like cloudbet, sportsbet.io, stake and recently roobet because they also launch there sportsbook as well. I only place bet on sites with good reputation here but I have no certain favorite.