In my opinion, it's not winning that counts, not really. It's about enjoying the game, and if your team loses because some random pubs really like spy, then so be it. The goal is that they will see that playing their favorite class is not essential to winning, and switch themselves. It always bothers me when people say in team chat, "Comeon gys, we dont nede 5 spies!!!one!!" when they are playing spy themselves. Instead, they should take the initiative to change to another class for the good of the team, or remain content with losing.
That being said, there is no shame in losing. If you lose the round, but are dominating all the enemy snipers from taunt kills, how is that shameful? I have been asked why I don't play competitive. The answer is exactly that, it's competitive. Winning or losing makes no difference in the greater scheme of things. All that that course of thinking makes you do is rage, and that's when you make the decisions that you will regret.
I realize I kind of went off topic on this one, so Swamper, to answer your question, a good heavy medic pair can carry a team to victory every time. Just ask Varil