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I own Commotion Pro v3, and it's outdated now due to a version upgrade, and an ownership change/buyout by Pinnacle Systems, but IT ROCKED for everything I needed at the time.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
Israel "Izzy" Long
Motion and Title Design for Broadcast-Film-DS