Do you mean the advertisment showing that the DVD was THX certified? If so, I like the one on the Star Wars Episode 2 DVD. I can't remember if I've seen it on the Episode 1 DVD because I haven't watched that one in over a year, and I don't remember seeing it on other DVDs.
My favorite THX demo is the one with the glass ball that shatters in a storm of lightning and thunder and then the pieces melt up from the bottom of the screen into the THX logo. It is simply stunning with an 16:9 HDTV and Digital sound system.
It's attached to the beginning of AOTC (and probably other films, but I can't recall which ones).
Lou that one was'nt on T2, it had it's own THX trailer done specially. The cow one was on either Toy Story 1 or 2 and is also on Monsters Inc. But I prefered the one where the THX logo breaks down and Tex (the tin man) comes out to fix it. It again was on Toy Story 1 or 2.
It's a shame that the Simpsons trailer was'nt on the Season 1 or 2 boxsets.