Personal opinion: A video recorder would be, by far, the best, but every one I've seen has battery life issues.
A digital audio recorder, even without an external microphone, seems to be the best bet until someone comes out with a DVR with a longer battery life that meets our needs for concealability. I would also love to see one that could be voice activated.
I use an Olympus VN-3200PC and the battery life is outstanding. It has a capacity of over fifty hours of recording. I use it in the sound-activated mode and generally have it either in my shirt or trousers pocket.
So far, I've only had one encounter that needed to be recorded and that was with an ignorant Wal Mart manager. Thankfully, and thanks to eye95 and others, the police and sheriff's department here are very knowledgeable.