whats the best action lens for a nikon d40?

Depends on how much you have to spend and what you are shooting and how close to the action you can get? The best sports lenses are the 300 f/2.8 and 400 f/2.8 which cost about 3500 and 5000 respectively. The next best thing is probably the 70-200 f/2.8 which is about 1100. After that, the 300 f/4 is also about 1100, then the 70-200 f/4 which is about 600, then the 70-300 f/4-5.6 which is also around 500 or 600. Last in line is probably the 55-200 f/4-5.6 and 18-200 f/3.5-5.6. If its just indoor sports you might be able to get away with the 85 f/1.8 about 400, or 135 f/2 about 1000. To get the proper shutter speeds, which for sports is 1/400 or higher when you can help it and no lower than 1/250, and f/2.8 aperture really helps. Sunny days you will be fine with up to f/5.6 or even higher at mid day, but when the sun starts going down, you move indoors, or its cloudy, f/2.8 is entirely necessary if not f/2 or f/1.8.