There is no simple answer to that and it largely depends on what you want from your TV and of course your budget. Here are some guidelines you can use to decide when purchasing a TV.

1) LCD, LED or Plasma?
First, Plasma is a completely different technology from LCD and LED is kind of a sub-category of LCD TVs.
Plasma TVs are superior to LCD TVs in several aspects. Experts say that Plasma televisions outrank LCD TVs in picture quality in both 2D and 3D, refresh rate/flicker, and price of course depending on the model. But they also lag behind LED TVs when it comes to energy consumption and certain features as far as durability goes. Current models of Plasma TVs are supposed to be much durable than ones from previous generations, however, you still have to take extra measures when transporting etc. because tilting the TVs are not advisable.
2) 3D or not?
This really depends on what you’re looking for, the price difference may not be as big as you think it would be. With a good deal price, you can get a 3D 46 inch LED TV for less than $1,000 these days. The price difference between a 3D and non-3D model may be as little as $200
3) Less than 30″, less than 40″, less than 50″ or over 50″?
This depends on where you want to keep the TV and how much room there is. If it’s going in your bedroom, you probably don’t want a 50″ TV 🙂
4) Refresh rate
LCD TVs are available from 60Hz, 120Hz, 240Hz and starting recently 480Hz models. With lower refresh rates, you may notice a slight flicker when it comes to moving pictures. The higher (faster) the screen refresh rate, the less you will notice this. However, it is the experts opinion that the average non-videophile user will not see a difference between 120Hz and anything higher. So there’s no harm saving some dollars by ‘settling’ for a 120Hz model in most cases
5) Resolution – 720p vs 1080p
Almost all newer LED TV models come with 1080p Full HD resolution while you will find 720p in some of the older models. Again, it is the opinion of experts that most of us will not see the difference between 720p and 1080p.
6) Energy consumption
LED LCD TVs have (as the name suggests) LED bulbs as the lighting source. In contrast, non-LED LCD TVs have fluorescent lighting. LED backlit TVs are more energy efficient compared to non-LED LCD TVs, and again, if you’re a light user, the saving may be negligible. However, going for an LED TV, even if it costs a couple extra $100 is advisable because it is healthier for the environment (green)
7) Internet ready smart TV with apps?
Again, largely a personal preference. If your DVD/Blu ray player is already Wi Fi ready and/or you’re using your Xbox 360 to access internet content, you may be able to bypass this and save some cash.

There is no simple answer to that and it largely depends on what you want from your TV and of course your budget. Here are some guidelines you can use to decide when purchasing a TV.

1) LCD, LED or Plasma?
First, Plasma is a completely different technology from LCD and LED is kind of a sub-category of LCD TVs.
Plasma TVs are superior to LCD TVs in several aspects. Experts say that Plasma televisions outrank LCD TVs in picture quality in both 2D and 3D, refresh rate/flicker, and price of course depending on the model. But they also lag behind LED TVs when it comes to energy consumption and certain features as far as durability goes. Current models of Plasma TVs are supposed to be much durable than ones from previous generations, however, you still have to take extra measures when transporting etc. because tilting the TVs are not advisable.
2) 3D or not?
This really depends on what you’re looking for, the price difference may not be as big as you think it would be. With a good deal price, you can get a 3D 46 inch LED TV for less than $1,000 these days. The price difference between a 3D and non-3D model may be as little as $200
3) Less than 30″, less than 40″, less than 50″ or over 50″?
This depends on where you want to keep the TV and how much room there is. If it’s going in your bedroom, you probably don’t want a 50″ TV 🙂
4) Refresh rate
LCD TVs are available from 60Hz, 120Hz, 240Hz and starting recently 480Hz models. With lower refresh rates, you may notice a slight flicker when it comes to moving pictures. The higher (faster) the screen refresh rate, the less you will notice this. However, it is the experts opinion that the average non-videophile user will not see a difference between 120Hz and anything higher. So there’s no harm saving some dollars by ‘settling’ for a 120Hz model in most cases
5) Resolution – 720p vs 1080p
Almost all newer LED TV models come with 1080p Full HD resolution while you will find 720p in some of the older models. Again, it is the opinion of experts that most of us will not see the difference between 720p and 1080p.
6) Energy consumption
LED LCD TVs have (as the name suggests) LED bulbs as the lighting source. In contrast, non-LED LCD TVs have fluorescent lighting. LED backlit TVs are more energy efficient compared to non-LED LCD TVs, and again, if you’re a light user, the saving may be negligible. However, going for an LED TV, even if it costs a couple extra $100 is advisable because it is healthier for the environment (green)
7) Internet ready smart TV with apps?
Again, largely a personal preference. If your DVD/Blu ray player is already Wi Fi ready and/or you’re using your Xbox 360 to access internet content, you may be able to bypass this and save some cash.