Unfortunately there is not "magic bullet" when it comes to web video, to many competing players. However, the H.264 codec has become increasingly adapted and is the current de facto format for web video. It will play on 90%+ machines and devices (some say 98%).
Encode using the H.264 codec and the .mp4 wrapper. Mac, PC, iPhone, iPad, Flash, Droid can all play this video encode. There are some browsers that will not play it because it is not open source, like FireFox.
For ideal results, you want to encode using the following recommendation:
http://camendesign.com/code/video_for_everybody
Alan Smith
Media317
http://media317.net