You're free to use the caption "best online casino" as part of your marketing strategy but be aware that this could also raise an eyebrow among members in the forum. Notice that more established casinos here don't use that. You should probably try something else.
There is no need for an established casino to use that tagline because they know that they are already one of the best. There is nothing wrong if they claim their self to be the best because maybe they have what it takes to become one? And they are not the first to use that marketing phrase but almost new casinos are also using it.
Not only casinos actually but the advertisements that we see on the TV, advertisements that we hear on the news and other places are also claiming their self to be number one if not the best. People are use to that so they are now ignoring it. They will still know the truth once they visit and test casino, products and services if its not a casino.