I agree that circle (sphere) should be used íf possible, since it easy and fast.
pixel precise, would be for a 2d game to test each pixel of the two collidion objects (bitmaps?) to see if there is a collision. although its quite nice to have this type of precision on the collision detection, I find that it is not worth it in most games, since the playser will never know the precission is that good anyway. obviously you should use circle or rect collision detection before you do pixel collision, if no collision in the first test, there will never be a collision in the pixel collision.
This applies also to 3d games, but there you use polygonal precise collision once the bounding volumes have collided. poly collision would test every polygon on the two object to find collision, roughly speaking. there several ways to do this i'm sure.
