The LDS one is very extensive, but can also be expensive and not as accurate as some might think. I have fought with a guy who posted a family tree on their site for years and they will not make him either take it down or correct it. I have sent him and them proof that he has my grandma married to her sister's husband and her sister married to her husband (my granddad.) They do have vast amounts of data, but each time they add a new one, they also find a way to charge more money for it.

I really like genforum for person-to-person queries and it's free. Also, there's Family Search, which is free and, if I remember correctly, is an LDS site.

A good place to start is Cyndi's List which is the most extensive listing of genealogical sites on the internet.