Not ever online casino that appears to be visible on the web is secure and worth trying. If you already have experience then you may already know about online casinos committing frauds due to which they get blacklisted also, the list of blacklisted online casinos can be found here: Blacklisted Online Casinos 2017 Online casino usa.
Now coming to your question, biggest online casino sites in USA then there are really some big online casinos which are reputed and on the same fair enough, for example Samuel has pointed out Bovada
Bovada is indeed one of the biggest and safe online casinos with millions of unique players visiting the online casino every month. Millions of users can’t be wrong, it in itself says a lot about the reputation and authenticity of the casino.
If you need some more then there are online casinos like:
Best usa online casino
And the list goes on, the terms and functioning of every casino differs, even the welcome bonus (which every players wants) is different, some end up giving huge bonuses and some just limit it to $300–500 which is not bad though. There are online casinos which like Wild Slots Casino which give about 50% every month on first deposit to players who have high funds and good background but UNFORTUNATELY Wild slots is not allowed in USA.
Never mind, you can try any of the mentioned above, all of them are genuine and authentic online casinos which are allowed in USA, make sure you check out their proper regions because some casinos like Planet 7 casino restrict certain regions of USA, the list of those countries is attached in the link which I provided, I found that review most helpful.
If you need more casinos then you can checkout this Online Casino USA List, it lists the most trusted, best and biggest online casinos which are legal and allowed in USA.
Hope my answer helps you and anyone else who is also looking for similar information.