After 25+ years climbing my own family trees and the trees of others, naming the "best" sites for discovering family history depends on who and what you're looking for. Ancestry.com is a good place, as brassfield says, which is why I have a World membership and have had practically since Ancestry went online. However, DO remember many of its family trees are compilations of information posted by many members and may be nothing more than conjecture on the part of the submitter. Stick to actual records...census images, birth and death indexes, etc.
USGenWeb sites are *sometimes* great sources BUT I've found many to be only sites with links to commercial sites like Ancestry, or else to the websites of the local genealogy or history societies that don't post anything except a phone number or mailing address so they can charge you a look up fee (or sell you one of their county history books!). There ARE a few USGW county sites that contain tons of information, but they are few and far between. It really depends on how active the gen or hist society is whatever area your ancestor was from.
Findagrave.com is a site I use regularly. Although not every grave in every cemetery is there, if someone has "walked" a cem where your ancestor is buried and submitted the information, the memorial page may have a pic of the tombstone AND/OR the obit from the local paper.
If all else fails, simply googling the an ancestor's full name sometimes gets me more info than I can find on any commercial site. Doing this, I've located obituaries in small newspapers as well as family trees published on "free domain" sites set up by a family member.