LCD TV in normally lit rooms. Plasma also can dim over time as well, though when new is very vibrant.
Something also to consider as well is a HD LCD Projector (not projection TV), you can easily get 100" wide on a pull down (or electric screen) for about the same price as a 40-50" LCD. I have a projector setup in my living room mounted from the ceiling, and its like having your own movie theater. You need the space and ability to control lighting however, so not for everyone.
Also stick with major good known brands like Sharp, Samsung, LG etc, they are worth the extra money and tend to be a lot more reliable.
Case you haven't seen this site its pretty good for home entertainment:
http://www.projectorcentral.com/