For part 2 of your question, "documentation of everday American life", I'd throw Mary Cassatt into the mix. It seems that artists sometimes go for the big ideas; Cassatt did a lot of work on mother/child interactions, surely the stuff of everyday life :)
Composer369 is right to mention Gilbert Stewart and, especially, Winslow Homer. Stewart is America's greatest portrait painter, IMO, and it's very hard to argue against Homer as our greatest artist. He was so versatile.
I would probably have gone with Norman Rockwell for my #1. But of people who haven't been mentioned yet, I'll put my two cents in for Edward Hopper. Nighthawks is one of my all-time favorites and I really like the sense of loneliness and nostalgia that most of his paintings of every day people and places convey.
Although his career was relatively short (less than 25 years), I would have to nominate Frederic Remington for this honor. No one depicted the Wild West better than Remington did during the latter stages of the 19th century. His paintings and illustrations of military and Western themes are among the greatest ever, but where Remington really stands alone is as a sculptor. He began this second career only during the last 13 years of his life, yet few Americans can be mentioned in the same breath as Remington. His famed bronze statuettes of western, military and cowboy life are still unparalleled today.
Winslow Homer's redefinition of technique and use of different mediums to express what he saw as reality is something that America and its artists aspire to in the modern setting. Like America itself, Homer believed in the idea of mutability in terms of artistic design. The ability for the artist to redefine themselves and their capacity is something that is critical to not only American art, but American identity itself. The idea of "work independently and solve your own problems" is not only something that Winslow believed about he and his art, but also something that is very reminiscent of America in the modern setting.
It is hard to determine things like greatest artist since art is different to each individual. I think though if you are talking about an artist who depicted everyday American life through his work you would have to include Norman Rockwell in that discussion. It is kind of sad today all magazine art seems to be digitally produced, it would be nice to see an occassional "real" artwork on the cover of a current magazine.
Greatest American Artist
Who would you consider the greatest American artist with regard to longevity of work and documentation of everyday American life?
This just may need a redefinition for as my first choice for this would be Louis Comfort Tiffany (1848-1933). Are you possibly only asking for greatest painter? Benjamin Franklin was quite the artsy guy also, yet also of the non-painterly sort, more applied art...we must recall he was a true Bostonian from 1706.
If the names Gilbert Stewart (1755-1828), Winslow Homer (1836-1910), and Thomas Eakins (1844-1916) are so overlooked as to have faded in the minds of Americans why do we have our history? This is a moot discussion, because their earlier American Realism is far from unapparent. What I am disappointed here is the very small remembrance of our first centuries as a Nation and how we have sequestered our concept of American Art as only the product of those living from 20th Century allowing some roots from the late Victorian. This seems irreverent to what America was created to be.Thanks Akannan for the nod to Homer !
My favorite is Norman Rockwell, because he captured the essence of what "Americana" is all about. He captured realistic portrayals of people and settings at their best and didn't rely on abstract concepts where you have to decide what the painting means.