Alien is arguably the best looking movie in my nearly 300 disc blu-ray collection. However, not all catalog titles have great or even good looking blu-ray transfers. My collection of the 1st 6 Star Treks movies is pretty mediocre at best.
I don't have too many of the traditional classics like gone with the wind, Casablanca etc but I am interested in getting some of them at some point, but I do own lots of older movies such as Tora! Tora! Tora!, citizen cane, and 2001 and they all look much better on my 65 inch Mits DLP then the dvd counterparts. I also have a good amount of 80's movies (lots of brat pack type movies and others such as legend, dark crystal, and labyrinth) and again they look better then most standard def versions, but again 5 star quality is not always the case. That being said I have been very happy with the overall picture quality that blu-ray offers over standard def content.
Audio quality on all blu-rays (at least most, there have been some bad apples here and there) has been superb, but one can only benefit from this aspect of HD content if you a have a decent surround sound set up and not a chincy htib or sound bar, or plain tv speakers. Listening to the tangerine dream soundtrack when watching legend for example just plain sounds awesome on my relatively modest 5.2+wides set up. Highly likely my living room theater area sounds better then when this movie was first released in theaters back in the early 80's.