From my experience, all of them have it own strength..but talking about the functionality and doing day to day job this is my rank.
Fusion 5.0 - strong compositing, real 3D particle, ability to import 3D, easy to handle (PC based), good support team at Eyeon, CC is cool
Shake 4.0 - strong architechure, fully customizable ( stime u need to add more node to archive sthing other sware can archieve, good scripting engine etc, good keyer/price. (Mac OX..i like it..)
Combustion - Average in overall..
For new user if budget is yr limitation, I recommend invest in Mac and buy Shake..later you can switch to any other node based sware cause its almost similiar.
Anyway at the end of the day only the compositor which will make the different..the sfware only the tool..we are the one who will craft the end result...So just select which one you use and make full used of it..