Given your premise this is just like aaking which is the better group of supporters of a football club. Since we have to consider them as a whole we can exclude the big one since there will be for sure at least one or more supporters who are ultras and will wank theor character to the heavens and or call names the people who disagrees woth them so for me of course it will be a fandom of a small side character (but not too mich otherwise there will be no fans at all) since people will not like him/her for her power, badassery or other flashy things but just maybe for their unique character and appearance. So I would say.. Franky, Perona, Chopper (if we esclude a certain someone), Monet... And many more.
Monet is the perfect example, I know there is a huge Monet fan in this forum, he/she of course is convinced that Monet will come back and join the crew, doesn't like if you disagree with that but other than that has "studied" a lot the character and even come up with a complex theory on how she will come back and join the crew. You can argue that fans like this exist even for other fandoms, and even that there are for sure more of this kind of fans in bigger fandoms yet in the big ones this is a very small percentage yet in this one, as far as I know and as far as we can say who is a true Monet fan, this fans rapresents the 100% of the fandom (or at the very worse the 10% which is still a huge piece that other big fandoms can not brag around).