This is so hard to answer. Are you talking about in an individual hand? Or are we talking about over the long run?

The best hand doesn't always win. The best player doesn't always win. The best player will win more than he will lose thus why he is the best. The best hand is only good in the hands of a good player. Frequently below average players (and occasionally the best players) lay down the best hand. That's what is so great about poker. You can force people to give up with you way behind. It's the ultimate underdog story.

At showdown though, the best hand always wins. Unless you having a stupid dealer, or play on Carbon. LOL JK.