I have been travelling in few of the countries and right now am in India, the most I hear on the television is about:

Parimatch
Betway
22Bet

In Ukraine the sites that were really popular were:

Parimatch
Bet365

In the UK it was more about:

William Hill
Sky bet
Bet Fred
Bet 365
Ladbrokes- Honestly their offices are everywhere

These are the sports book that I have heard off/ seen being advertised/ saw the offices everywhere