Quicktime Player is great, but it still seems to struggle with certain formats without installing extra codecs etc. The best Mac App that I've come across for video playback has to be VLC Player from VideoLAN. This App is completely free and performs amazingly well. It's a little gem. In my opinion, if VLC Player doesn't play it, well then it probably can't be played on your Mac.
The interface is clean and tidy, uses drag and drop and provides all the controls you need. Personally, I keep the VLC Player icon in my dock and any movie file I want to play, I just drop it on VLC. Just look for the Traffic Cone!