*There was no category for Little Miss Sunshine*
Everybody seems to have forgotten about the how the experience of Edwin Hoover (Grandpa) on the road-trip adds to the critique of American life.
Grandpa was a foul-mouthed, heroin-snorting horn-dog with a general distaste for his son Richard's smugness and air of superiority. The character serves as a commentary on an elder looking back at life with some perspective and determining that in life, he did some things the wrong way -- so he insists upon imparting his knowledge back to his family.
Thus, we have the lines, "I got no reason to lie to you, don't make the same mistakes I made when I was young. Fuck a lotta women kid, not just one woman, a lotta women," as he talks to teen Dwayne on the road-trip. These lines imply regret concerning how Edwin obeyed the sexual norms of his time (not a lot of sexual partners). Additionally, when questioned about why he snorts heroin, he says when you are young, you are crazy to do drugs, but, "When you're old you're crazy not to do it." When he was young, he had the energy to live life, but he 'wasted it.' Now, in his older days, the heroin gives him the energy to do the things he needs to do. Energy (and zest for life) is something that one generally loses as one gets older, but because of the heroin, Edwin seems to have it in spades.
Grandpa does have a softer-side to his gruff exterior. In the van after Richard has learned of the failure of his program to launch, Grandpa (who has been very vocal about his distaste for Richard's plan) admits that he finds it brave of Richard to take a chance on his dream (which is something he never did). There seems to be a bit of wisdom splashed with regret in this comment in that he never took that chance. Also, when Olive wonders whether she is a 'loser' or not (per Richard's comments about winners/losers), he says, "Losers are people who are so afraid of not winning, they don't even try." This, of course, sets Olive at ease.
This comment about losers being people who don't even try could be considered the overall theme of the film. One is a loser if he or she is so afraid of not winning that they are paralyzed, so take a chance, be yourself, and (to quote Dwayne) 'fuck the rest'. Edwin only began to live by this philosophy at the end of his life, but by the end of the film, the entire Hoover family adopted it.
Somehow, I think Edwin (wherever he may be) would be proud that his family has let go and embraced life. :-)