The GC controller is insanely overrated. The D-pad is made for people with pencils for thumbs, the overall feel is cheap.
It's pure nostalgia. It was a real improvement over the N64 controller in a lot of ways, but still an awkward piece of work. The Wii Pro gamepad is far far FAR FAR FAR better.
Basically anyone who grew up with (insert controller here) as the controller they used most during their formative gaming years will say "Oh, it's the best!", when in fact it was just something they got used to.
Better controllers objectively than GC :
NES, SNES, TG16, Hori Commander Series, Wii/WiiU Classic/Pro Pads, Xbox OG 'S', Xbox 360, DS2, DS3, DS4, XBO, etc.
EDIT : Forgot the DreamCast pad, which craps all over the GC controller as well. Silly me. Sega pads were usually awful, but the DC one was outstanding.