The oldest known Touhou fan animation is this https://www.nicovideo.jp/watch/sm1483, made by Shiobi for Touhou Saimoe to support Yuyuko for the second Touhou Saimoe, in 2005. Yuyuko did not win, sadly.
The most popular meme is Touhou Yukkuri. So popular it has become that it has transcended the series itself, spawning as its own genre, with even random japanese grandmas and preschoolers knowing what it is, even if just by osmosis. You can find thousands of videos made with Yukkuris daily on Youtube, just search "ゆっくり".
Not counting endings, Reimu has technically appeared in more games than Marisa, with her appearing HRtP and Gold Rush but Reimu only didn't appear in GFW... but Gold Rush is most likely non-canon, so actually, Reimu and Marisa are tied in game appearances. Counting endings however, yes, Reimu does appear more than Marisa, as Reimu appears as a cameo in GFW.
Marisa, of course, is the icon of every mainline Touhou windows game, making her the character used most as the .exe icon. Surprisingly, she also appears as an icon for Shoot the Bullet, for some reason.
Marisa and Reimu are tied as the character with the most themes, both with eight.

More to come when I feel like posting more.