LED is LCD. It is just a specialized form of LCD. Regular LCD TVs use a CCFL backlight. LED LCDs use an LED backlight. In terms of quality, that's the difference between them (they also differ in power consumption and thinness, but that doesn't impact image).
Plasma will give you better color, contrast, and blacks. Plasma has a near instantaneous response time, which means it plays back all forms of fast motion flawlessly. Plasma is a better image experience through and through. LCD doesn't have a single option that can beat it on any level.
Plasmas suffer from a lot of myths and misconceptions though. They no longer suffer burn in, they do work in high altitudes, and they no longer have short life spans (they can easily last the same or longer than LCDs). Also, they are no longer necessarily power hogs. You can get energy star certified Plasmas now.
The LED TV is better as it is used less back-light power. It has the same technology as the LCD apart from the back-light specifications but it is a better alternative. It also has the advantage of being more evenly distributed throughout the screen. Another feature that makes it a better option is the fact that it is brighter and improves the contrast ratio as well as the color.