I use caloriecount.com...It's the only one I've ever used, so can't really compare it to the other websites.
It works quite well for me. It is fast, and has plenty of features to make frequently eaten foods easy to select (you can tag them, tag whole meals of different foods to add at once, and it offers a selection of most recent foods for each meal). It seems to have all the brands/foods that I eat, and several accurate information on restaurant meals. If not, adding foods is an easy process. It also has a recipe analyzer that is really handy. You just plug in your ingredients and the amounts and the serving sizes, and it gives you the analysis per serving. Then you can save the recipes (with instructions and such). The analysis is pretty user friendly and informational. I only make sure I have enough protein and fiber and have my desired calorie amount, so I don't pay attention to it too much otherwise.
One thing that annoys me, and it's REALLY not a big deal...you enter your target calories...mine is set at 1500...and your "calories consumed" bar stays green as in good as long as you're significantly below that number. So if I had 700 calories entered for the day, my bar would be green, even though 700 cal is way too low. As I get close to 1500 cal, the bar turns red as in bad. That annoys me, but I just ignore it.
I also avoid the CC.com forums. There's some good discussion and information, but I have to wade through too many teenage girls posting pictures of themselves at like 110lbs asking "am I fat???" and other such nonsense. I just use caloriecount for tracking calories and come here for my forum needs