PandaNiku's prominent posterior doujinshi predated the Attack on Titan anime. So, for a while, they were the most popular Titan images on Japanese art community Pixiv. Pics of Sasha "Potato-tan" Blause are certainly popular, but might have peaked. However, it's the fan girls who have decided Attack on Titan's most popular tribute art.
From last July, an early favorite, now with 1040 user nods
Probably the top Sasha Blause work, approaching 9,000 nods
But, by a commanding margin, the queenTitan fan tributes is yaoi artist Mekabu with her pictures of the male characters adjusting their harnesses.
Series at 7.000
Mikasa trails at a still very popular, but far lower 3,300