Probably the most important thing is your budget.
No budget - Plasma is the best, look for $3,000 or so for the cheapest, most are closer to $5,000 or so. Prices won't drop too much but more things will be added such as built in HD tuners which will save you money.
Very expensive - LCD or LCoS (liquid crystal on silicon). Not quite as good as Plasma but still thin screen, almost as expensive.
good to great - HD rear projection tv's. Prices have come down so that price range is in high $1,000's or so to mid $2,000's. Widescreen is the way to go as even if you watch regular tv newer models do a good job.
ok - regular rear projection tv. If you want to save money this is the way to go but you will be sacrificing high quality when watching DVD's. Low to mid $1,000's
Since I had bought a nice 36" regular tv not too long ago, I ended up buying a projector (~$1,200) and a pull down screen (~$250) so now I can watch movies at 110" that look great. The room must be real dark though so I only use it at night. Best bang for the buck when you want a real big screen.
Of course, I hope you have a nice surround sound system to go with the tv. If not, I would get the cheaper tv and upgrade the stereo. Of course, this mainly applies if you are into movies and stuff.
If you have a budget range, post a reply and I'll recommend some good models and features to look for.
-murph