Go and try them and then buy the one you like.
I've only tried the iPad so I can only speak for that (and anyone criticising the others without actually having tried them is silly) but it's a brilliant piece of kit. Light, supremely easy to use (my 76 year old mum can use it with no problems!) and you have the biggest app store with a plethora of free and cheap apps to cover pretty much anything you could want it for.
Transferring videos is simple. You can rip them with a free bit of software called handbrake and they show up nicely with chapter thumbnails. The screen is very clear and the controls simple. Email, photos, web pages, google maps etc all work very well.
I'm sure all the other tablets do similar jobs, it's just a case of which one you like the look and feel of.