There are very few lenses that I would label as "flawless," but the Canon 70-200mm f/4 L is one of them. For about $575 US, you can get a solidly built "L" zoom that is tack sharp at all focal lengths (center and edge sharpness), even on the most demanding sensors, such as the full-size Canon 5D.
I know many pros prefer the f/2.8 version of this lens because of its light gathering power, but that lens costs twice as much and is much heavier and larger. Photographers on the go who want the best image quality possible, and can live with f/4, will be thrilled with the lighter version.
Last year, Canon also released an Image Stabilized version of this lens for about twice as much. Not only do you get the same great optical performance as the original f/4, but you get up to 4-stops of stabilization. You can find out more by reading this review on SLRGear.com. This is really the only way to improve upon the original.
Either way, depending on your budget, the Canon 70-200mm f/4 is one of the most pleasurable lenses you'll every shoot with. And if that isn't good enough, the pictures it produces are outstanding. Keep your eye on the used market too. A lot of the non-stabilized lenses might be showing up as fans upgrade to the IS version.