Not nessicarily the "best", but like any 3D application, there are 2D compositing apps that do different things.

So, it really depends on what your shot needs.

I personally like After Effects, but I'm thinking of getting Combustion when I get the money.

The demo just blew me away

I own Commotion Pro v3, and it's outdated now due to a version upgrade, and an ownership change/buyout by Pinnacle Systems, but IT ROCKED for everything I needed at the time.

it's got a GREAT tracker, and roto-splines that are so good, that the now-defunct MILL FILM used it exclusivly to roto, and composite all the shots for THE GLADIATOR.

If you have compositing-specific questions, one of our newest members, Paul, has a compositing forum site called: VFXTALK.com.

Talk to Hugh, he is thier resident After Effects guru

Check SM's links page for thier link