Ipads, Windows Tablets, iphones, Android Tablets, PC, Mac - which is the optimum cross-platform file type to use when providing clients with downloadable clips from my own cloud and not a commercial service.
At the moment (March 2014) I'm thinking MP4 720p but i don't know. Anybody want to chime in?.

It depends on what the purpose of the download is. If I am showing a project to the client, I upload the same MP4 file, 720P, to my Google Drive and provide the download link. It works just as Youtube does, it adapts download speed to the actual internet connection, and of course Youtube apps are available for all platforms.

If I deliver finished product, I deliver what the client asked for.