What are the most effective styles of kickboxing/striking?

A style is only a tool and only as useful as your abilities to use it. It all depends upon your instructor and training.

People are effective, not styles.

I know you're looking for a hard, fast answer, but a crap [insert style here] guy will always get handled by a [insert style here] guy at the top of his game.

You're never going to get the "right" answer. You're going to get guys talking about their own experiences and preferences. Asking which is the better style is like going on a gun forum and asking which is the better pistol, Glock or Beretta or something else. There's going to be guys trying to prove their gun is better than the other guys. You have to look at what schools are around you and take a couple free classes from each and decide which one is your preference. Hope this helps and good luck on your martial journey!

The one taught by the best teacher.