You know, if I was in chat as often as I used to be... or at all for that matter, I probably would have bored you with recommendations. Also, I can't find a link for it at the moment (and those ^ wont load at work for some reason, so I don't know if it's them or not) but I know there's a dedicated website that focuses a whole helluva lot on the quality of new BluRays, and comparisons on how they stand up against their DVD counterparts. I'll see if I can find them name and post it later.
Anyhow, if you happen to have any form of personal rating system for the films you like/watch, start with anything that you would equal to a glowing 5 star review. If you have a truly great film (good place to look
), it's always well worth it to see it in the best possible quality.
Off the top of my head, of what I can recall of my brothers collection that are well worth the price, and look great in HD.
The Shawshank Redemption, Black Hawk Down maybe, just about any film made by Pixar, or original Disney films, Casino Royale, Batman Begins and TDK, 2001, Terminator 2, Iron Man looks pretty amazing... might add more later.
If you really like Gladiator, I know that a second run has, or will be, started on the BluRay as the quality of the first released was so under par.
Most importantly though, as Draykov said, go with your favourite first, and from there chances are you'll be able to find out how well it got HD-ified.