If you go by the reviews, Star Trek and Forrest Gump.
I've got both. Trek is very nice, but Gump, which most proper review sites say is one of the best quality releases on Blu-ray ever, ain't all that IMHO. It's miles head of the DVD in picture and sound quality, but suffers from too much picture softness on wide shots. Plus there's a fair bit of speckling on the master in places.
Leon and Transformers are also excellent.