Do you mean lyrically? Eskimo Blue Day was hard, and We Can Be Together was REALLY hard. Then again, The Ballad of You and Me and Pooneil (my favorite) is hard at times. The bridge that Paul Kantner sings is hard because the Grace part is so cool-- It distracts from the actual lead vocals, so it makes it harder to learn
Also, Rejoyce can be tough to memorize, just because so much of it is a giant non-sequitor. Don't get me wrong, I love that song.
Trouble in mind, lord I'm blue
But I won't be blue always
The sun's gonna shine in my door,
Shine in my back door someday