If I had to pick ONE website, it would be Ancestry.com. They aren't the least expensive, and familysearch.org is certainly getting closer to bumping them out of that "#1" spot, but Ancestry.com has so many great records that I can't imagine working on genealogy online without access to Ancestry.com
I have been a subscriber for 13 years or so, since it was a new website. While I don't always agree with things Ancestry.com does (such as the ending of their Expert Connect program), they continue to add content that I am delighted with all the time.
I also really like the Member Trees, because I can add records directly to my online private Member Tree there.
One really does have to watch out for false "shaking leaves" (ask my family who often hear me moaning about others who have clicked those "shaking leaves" until their tree is nothing but nonsense) but the hints are often useful. Perhaps not as useful as the Ancestry.com commercial imply, but if you are careful about reviewing the hints and not just clicking each one and assuming its correct, they can be quite handy.
I am partial to the record sets that have images, and always review the actual images of the documents to read for myself what the record says.
One weakness is their indexing is clearly not always well done. I correct many census indexes nearly every day I work from Ancestry.com records. Still, for someone like me, who has worked on genealogy back in the "old days" when there were no census indexes for most places, and you would have to spend days or weeks pouring thru microfilms *hoping* to find the family you were looking for, it is a godsend to be able to work on those same records from home, late at night, sipping hot chocolate in your PJs.
Ancestry.com will be entering the autosomal DNA testing market this year, which could bring even more greatness to their site.