The Criterion collection blu-rays are awesome. Here is a list I found that lists 50 of the awesome-est.
The 50 Best Criterion Collection Releases - Film.com
I've only seen a couple of the films on that list so I'm going to try and see some of them.
One of my favorite Criterion Collection films (at least in terms of biggest improvement) is the 1960 Lord Of The Flies. I had seen a supposedly restored dvd version of this film a few years before and the Criterion blu-ray was 1,000 times better. The dvd version was washed out looking and the dialog was hard to hear. When I saw the Criterion blu-ray, I could not believe I was watching the same film, it looked fantastic and the sound was crystal clear. I have several of the Criterion collection films and I love them all.