I know this isn't going to help you much, but I've tried all of the F2P MMOs around, or so I have myself convinced, anyway.
Risk the loss of some money and buy Guild Wars 2 if you want an extremely unique, addicting, and enjoyable MMO experience.
As I said, I have played so many F2P MMOs in my time, and I have also played many P2P MMOs in the past ten years or so, the best one I have ever played, by far, is Guild Wars 2. The digital edition is on sale right now, and can be bought for 39.99 USD at the moment. This is 30% off the regular price of it, I believe.
If I were you, I'd throw that money to Arena Net and get Guild Wars 2.
But, if you really do NOT want to spend any money, what type of MMO are you interested in? F2P, yes... But what type of genre? Sci-Fi? Fantasy? Etc...