I don't think of competitions as scams, so much as a waste of time and money.
The ones that are worthwhile are the ones that you see on the covers of the paperback editions of books: National Book Award, Pulitzer, Golden Duck, etc. Generally speaking, these are for already-published books, and, again generally speaking, the entrance fee is paid by the publisher.
If your work is good enough to win a contest, it's good enough to attract a publisher.
Saying that your book won a contest won't make a publisher more likely to buy it, though.
If you're just doing it for fun... look at the list of previous winners. Have you heard of them? Have you read them? Were they any good?