Stock manifolds are generally NOT designed for performance. They are restrictive and have lots of bends, which reduce noise and "tune" the engine to sound a certain way.
Depending on the engine you have, you may be able to purchase and install an aftermarket manifold. For example:
2.4L Twin Cam motors have several options, including swapping over to a modified 2.3L Quad 4 "High Output" aluminum manifold.
2.2L Ecotec motors have a racing manifold available from GM.
2.2L OHV motors have ported manifolds available.
Some users have even designed and built custom manifolds, for a variety of applications. As far as the "best" intake manifold available? For the price, usually most owners will stick with the stock manifold as it is generally less restrictive than the rest of the intake system.
