I have used RaffleCopter, but for competitons on other social media for my past projects (such as Twitter).
It seems to work really well. You can set how many entries a user gets by each action (i.e. tweeting a message, liking a page, following etc.)
Don't think it has a "YouTube subscribe" option though (if this is what you want).
But it's useful for building the fanbase for your channel on other Social Media.