If price is not an issue, then I will have to say plasma, by a very small margin, has better picture quality.
Plasma is a mature technology, while LCD flat panel still has room to improve. And given the latest models by Sony, Sharp and Toshiba, I will say LCD is a worthy rival to plasma in terms of picture quality.
Personally, I will pick LCD over plasma, since I think the picture quality of the best LCD TVs out there is good enough for me, and the slight disadvantage outweighs the issues with plasma.
Plasma TVs can not avoid the burn-in problem, which will come up sooner or later if you watch 4:3 pictures without zooming. The 2 black bars on the side will age less than the center portion, in other words.
Now this problem of plasma is probably exaggerated by LCD TV makers, since it might be years before you see the difference in aging depending on how you use your TV, and 4:3 programs are slowing being phased out in the mainstream.
The overall aging of plasma panels is also faster, but it might not be an issue depending on how long you will keep your TV. The other small issue with plasma is that they use more electricity and can get pretty hot.
Lately Panasonic produced a new model with anti-reflection coating on the screen. The reason is about another issue with plasma: they are glass-based and hence not too good in well-lighted rooms.
Now plasma TVs do have better picture quality due to its better contrast ratio and deeper black level, and they are notably cheaper, especially the 720p HDTVs. 1080p plasma is quite expensive too, although still cheaper if compared to best 1080p LCDs, but more expensive than 1080p LCD TVs from value brands, which picture quality is really sub-par.
The whole industry is moving towards LCD flat panel.
Anyway, the best LCD flat panel HDTVs are from Sony, then Sharp and Toshiba, Samsung is pretty good too since they use Sony’s panel. Don’t go by other brands if picture quality is your top priority.
The best plasmas are from Panasonic and Pioneer.
If price is not an issue, go with the latest models from these brands.
By the way, plasma has size of 50 LCD HDTV, but LCD has 46" or 52", usually.