I suppose you need a large carrier for a large message to be hidden. Very high resolution home movies maybe? I assume you can't use content that is already public, as then the original can be compared with the modified version to prove that hidden data exists.

porn. It gets transcoded enough that the stego could fly through as just another lossy transcoding. Better yet, lossy transcode it to something and back, then stego, as long as your target has a copy of the same transcoder and knows what you're using as reference. You should also be careful to strip metadata as to which transcoder you're using. They're PETABYTES of the stuff floating around.

And if you're willing to pay a few bucks for private cams, there's no way anyone else can have a copy of your screencap, so no amount of clever transcoding will get them a reference copy.

Porn is actually quite a good one. It's something that doesn't look out of place if hidden away, and gives some plausibility to owning the stuff. More so than having home videos and no family, for example.