Like someone said previously, I have also never ended a relationship with my best friend. I have however lost one, and it hurts much worse than losing a significant other. Because a best friend is one who you can talk to anything about, and are the ones there for you if you ever DO break up with your boyfriend/girlfriend. And if you don't have that, then you have lost something essential to helping one cope. When you lose a boyfriend/girlfriend, there are still other people out there willing to fill that position. But a best friend is difficult to come by and is based on years of understanding, trust, and closeness.
My vote's for best friend.