For me, the worst anime that I had watched was NHK (i know i am going to be blasted for this) but I found the whole artwork and the premise to be rather disappointing. I had high expectations of it, which was probably why it failed so badly for me. However, many of my anime friends (including the ones that are blogging with me in the animeblogger.net blog that i have) all loved NHK. So, i guess i just have weird taste.
For the best series, i will go with yet another controversial choice, Bokura ga ita. I found myself liking this slice of life /school life/romance anime a lot because of the portrayal of the characters and the development. It's nicely paced, with a simple but beautiful artwork.
I must say FLAG and Chevalier might be good stuff. Can't wait ...