The best wingman is the guy who has been on the scene recently. He knows who is around and, more importantly, knows what you like. He keeps his eyes open for "your type" and is quick to alert you when she pops in. If he speaks Spanish it is a definite benefit and if he has boots-on-the-ground connections it is even better.
A really good wingman is enough like you to roll in your favorite venue yet different enough that he is not after the same girl that you like all the time. He keeps one eye on the room and one eye on you so he knows where you are and anticipates when you are ready to move. He is flexible and can roll with a changing situation. He is timely and does not keep you waiting. I always know where he is but I don't have to rescue him or hold his hand.
He knows when I am hooking up and I don't need to say anything about it. All I have to do is catch his eye and give him a nod and he knows he will see me later or maybe tomorrow. He can roll on his own as well as I can so it is never a problem. He is unshockable.
You would think good wingman are rare. Fortunately, thanks to CRT and SL, they are fairly plentiful!

Well...well said. Also if it ever goes down in the streets a good wingman wont look for the next taxi to dive head first into. A wingman is good for numbers out in the streets in case you have to apply a beat down
And everything Pac covered above with the exception of a wingman being handy for translations if he knows the language.