Unless we get a Windows 8 Pro tablet all other tablets have limited video decoding capability. The iPad is slightly better in that it has VLC Player http://www.videolan.org/vlc/download-ios.html writes:
"VLC for iOS can play all your movies and shows in most formats directly without conversion.
You can synchronize media to your device using WiFi Upload, iTunes, Dropbox, or direct downloads from the web."
Your post makes me think you might wand the iPad, Windows 8 Pro tablets because for Android most folk explode on the launch pad as the video does not play without conversion.
Beware the Android lovers that will play this down.
I'm looking to buy a tablet where I could watch movies online and offline, and if possible transfer movies into the tablet using usb or something.
would appreciate the help.