What is the best universal video format / extension so that it can be viewed on iPad and PCs
h.264 is very widely supported and can be found in multiple different types of container formats.  mp4 or m4v are probably the most widely used extensions, but particular support for files varies based on what players are installed on devices.

MP4 is the official MPEG 4 standard container format.  M4V is an originally Apple based container which also added AVC support, but is still a bit more open than MOV.  Most players will also interchangeably play a file regardless of an M4V or MP4 extension provided that they support reading both file formats.

MP4 probably has slightly better overall compatibility while M4V probably has slightly better compatibility on Apple devices, but that is also subject to change and is hard to measure accurately.