I have to echo the above. USA SOng comp. seems legit. The John Lennon Songwriting Contest is also pretty cool.
The question here becomes; why? Very few people end up getting listened to by anyone who is "anyone." Yeah, the money and prizes are cool, but the clout of winning isn't going to get you a contract. I've entered JLSC twice, never heard anything back. I think the USA song Comp. will send you reviews which is cool and could be worth the entry fee. BUt there are other ways to get reviews, exposure, and such.
Just ask yourself why you want to do this.