best: An old African man grabbed me and yelled "come on boy run harder, run like a lion!" it was the 2nd last marathon mile, i was hot, dehydrated, exhausted and comming in near last :) i sprinted two miles, energised and ferocious. i use that moment every time i feel done.
worst: same race, i was walking during mile 18 and a volunteer yelled "your doing great!" I replied "The heat's gotten to you and made you crazy mamm!" :) i think of her whenever i see ppl cheering on those who will for sure lose. that last thing a marathoner wants to hear is how great they are, especially when they are to place last :)