Just my two cents: I personally use VLC most of all. Mplayer will play for me what VLC won't play as well (for example, corrupt .mpegs) and I keep Quick for .mov and .mpegs I know for a fact will play well in it. It has after all the most direct controls of the three.

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iMac G4 OSX 10.3.9
RAM 256MB
LW 4.10.5 Basic
ADSL anything from 3 to 8Mbps/around 1024kbps

"Raise your can of Beer on high
And seal your fate forever
Our best years have passed us by
The Golden Age Of Leather"
-Blue Öyster Cult-