Where can I watch all episodes of Ben 10?

I think you can watch every single episode of Ben 10 on You tube. It has a dedicated Ben10 channel. You could try to go back to the very beginning and enjoy the series from the start of Ben's story. It is nice to see where he finds the alien pod and the alien artefact leaps onto his wrist.

All the following adventures come from that moment, really. Ben starts transforming himself, and we learn what possibilities there are and soon all the aliens show themselves.