In my experience, plasma is the best way to go and Panasonic is the best of the brands as far as picture quality and reliabilty. Pioneer is also known for great pictures, but they've had a long record of making electronics a little on the sub-par. I own one Pioneer component now (a CD recorder) and it's noticeably clunkier than other DVD/CD players. The bad thing about Panasonic is they make about 1500 different plasma models. The model designations make it very difficult to comparison shop, but you should get a consumer model, not a commercial model. If you subscribe to cable or satellite or have an over-the-air HD converter box, you can save some dinero by getting a model without a built-in tuner.
I'd stay away from Toshiba, just because of quality control concerns. Two years ago I bought a Toshiba laptop (model A135-S4447). Within 3 weeks (of not a whole lot of use), the backlight went out on the display. I took it back to Office Depot and they replaced it with a new one. As soon as I got that one home, I noticed the touchpad wasn't working. I took that one back and they gave me a new one. I should have checked it at the store - several keys on the keyboard didn't work! When I took it back, they had no more in stock of the same model. I paid $150 more and took a Sony home. I've had no problems with it. Perhap's Toshiba's TVs are problem-free, but I think if my laptop experience is any indicator, they've got some horrible QC problems.
I've also had problems with Mitsubishi TVs, although older CRT models. The problems were in the circuit board connections. The solder joints would periodically fail, requiring a $150 repair bill every 3 or 4 years.
Projection TVs (DLP and some LCD) are much cheaper than plasma and LCD flat-screens, but have their downsides. The lamps periodically burn out and aren't cheap to replace (the last I heard, about $250 a pop). Some people have complained about rainbow images from DLPs, too. Projection TVs are best viewed in dim lighting, so if you watch TV at daytime in a room with a lot of windows, the picture will look a bit washed out.
LCD flat screens are good for rooms with a lot of light. The reflective characteristics of the screen are good, so you won't see reflections of your furniture as much when you're watching them.
Plasma screens have a bit more reflection than LCD, but one of the best things about them is their wide viewing angle. If you've got a gang of people over, almost anyone who can see the screen will get a good picture. LCD flat screens have a much more narrow viewing angle. Plasmas are less energy-efficient than LCDs. My current 65" Panasonic runs about 800W, which is the equivalent of burning 8 100-watt incandescent bulbs. I don't watch a whole lot of TV, so it's no big deal to me. Plasmas also require a little more careful handling. If you get fingerprints on the glass, they recommend that you don't use water or liquid cleaners to get them off. It's tough cleaning them off without something liquid. Also, as the main cost of an LCD or plasma is in the actual display, damaging it during mounting or other mishandling means your investment is basically worthless. Damaging the screen of a projection TV should be a much, much cheaper repair!