Are we going by personal bias or more concrete stuff? I'll list the ones I think should be up there for less subjective reasons:
>Nousagi, Yukimin, Haatons, and Fandead
The only fanbases where I can say that the fanbase representation gets consistently added into a significant portion of fanart in more than a comedic way. Takos, Sukonbu, Koronesuki/Listener-san, and Nekkos are seen somewhat often, but not as often as those four, and almost always are used for a gag or joke. In order of appearance of the four it'd probably be Nousagi>Fandead>Yukimin>Haatons
>Nousagi and Fandead
The only two fanbases to appear in Holo Alternative
(Rushia is weird here because it used the OG depiction of Fandead but she is a necromancer anyway, and it's not like people miss what it's supposed to be)
>Nousagi and Yukimin
The only two fanbase representations to actually make it on their oshis model. This is fucking huge, Nousagi get to literally sit on Pekora's head and Yukimin get to literally bury themselves in Lamy's tits
So overall I'd probably say Nousagi are first, Fandead and Yukimin are basically tied for second but if I had to give it to one I'd give it to Yukimin simply because being visible on your oshis model and having her play around with "You" is probably one of the greatest things that can happen to a fanbase depiction.
