"LED" TV's are just LCD panels with LED back lights. Lots of individual LED's can give the electronics in the TV better control over the back light. For instance, dark parts of the picture can be dimmed locally to help improve contrast. LED is going to be the most power efficient and produce the least amount of heat.
LCD TVs have fluorescent backlights. There are tons of LCD panels out there, so they may be a little cheaper than an LED panel. The fluorescent back light will use a little more power and generate a little more heat than LED.
Plasma is another technology completely. It is the least power efficient of the flat panel TV types and generates the most heat. Older plasma TVs used to need to be recharged ever ~5 years or so, but I think they got that worked out. Plasma will suffer from burn-in even worse than a traditional old tube TV would. Plasmas are cheap because there is less demand for them and the technology in general has come down in price.
LCD/LED TVs are great in bright rooms, and plasma excels in a darkened home theater environment where the lighting is very controlled.
Plasma has a very high refresh rate and response time, so motion is usually very smooth and clear.
In my opinion, an LED TV's strike a great balance balance with high contrast ratios so they can be used in bright rooms or dark home theaters. Any TV will need to be adjusted differently for each environment though. I also like the lack of burn-in on LED and LCD panels. There are many LCD or LED panels out there with 120Hz or higher refresh rates. 120Hz is the key because it is a multiple of 24, 30, and 60 frames per second. These are the three frame rates that you'll find in TV, DVD, online videos, video games, and blu-ray content. 24 frame per second content on a 60Hz panel will have to go through some funky frame rate conversion where some frames are shown longer than others. This causes judder that drives me nuts personally, but many people don't even notice it.
There are a lot of TVs with "240Hz" refresh rates advertized. These sets have some active processing that take the incoming 24/30/60 frame per second signal and interpolate or synthesize new frames to go between the actual frames. This helps to smooth out and sharpen the video. It works well for some types of content. For other types of content, it may actually make a film source like a blue ray movie look like a soap opera shot on video. This is sometimes referred to as the "soap opera" effect. I turn all of this active processing off because I don't like the effect. My parents on the other hand don't seem to mind it though and like the extra smooth picture.
I love Samsung TV's. They are my pick. Samsung makes panels for a lot of other company's TV sets and computer monitors. I would also consider Sharp, Sony, Visio, and LG. Panasonic has been known for really nice Plasma TV's.
You can check out the reviews on cnet.com. If they don't review the exact model you're looking at, they usually have one in the same family of models that might give you an idea of quality.
For a bedroom TV though, this may all be way too much info. Even a cheap TVs made today will look as good as high end models from 5 years ago.
-SF