Here are a few examples:
Fernando Gonzales - monster backswing loop, monster forehand:
Juan Martin del Potro - medium backswing, still a big forehand:
Andre Agassi - more of a straight takeback, due to the fact that he was usually hitting the ball on the rise, at shoulder level.
Agassi is usually used as an example of a pro with a shorter takeback, but if you watch more videos of him playing, you can see that he also had the tendency to do a loop when he had more time to prepare for the shot.
The general trend is, the bigger (more loopy) the backswing, the more powerful the shot.
At the park, I see all the time guys with a really short takeback, simply lowering the racquet at knee level behind them, and then just hitting up from there. But this way they "muscle the ball" instead of using the swing of the racquet to generate energy, and they can't really put enough power behind their shot.
Personally, I like Del Potro's swing. It is a relatively simple, clean motion, and is easy to imitate.
But when I have more time to prepare for the shot, I still go for a Gonzales monster winner!