Funk not appearing in a lot of GOAT threads on here shouldn't be indictive of his quality, he's just a victim of circumstance in that so many people on those lists are either Attitude Era stars or prior stars from the Golden Generation or before that, who've become enshrined in any 'GOAT' discussion through either their starpower, charisma, character or drawing ability.
Make no mistake about it, when you remove all the booking advantages, backing from the higher ups and the opportunity to become a major star, and instead focus merely on who is purely the best 'wrestler' (ring work, character work, getting an angle over, storytelling, selling, psychology, a general performance) Terry Funk absolutely belongs on any GOAT thread. The reason he is overlooked is that younger fans aren't brought up/encouraged to watch his prime years and are instead spoonfed to worship the likes of Hogan, Savage, Austin, Rock, Angle, Michaels, HHH, Foley etc. That isn't to say those guys I've just listed aren't worthy of being praised, its just that whether its WWE DVDs, youtube highlights of visuals that will stand the test of time (Hogan slamming Andre, Macho and Elizabeth reuniting, Austin 3:16, Backlash 2000, Montreal Screwjob, HIAC 1998) those guys are automatically the go to guys for a majority of people on here as to what they consider 'the best': though of course this usually takes into account additional attributes like drawing power, charisma, mic work, quality of feuds, length on top etc.
If you took a GOAT discussion and focused simply on ringwork and its additional attributes, Funk would absolutely make any list alongside Flair, Lawler, Windham, Rhodes (Dustin), Arn Anderson, Regal, Mysterio, Finlay, Steamboat, Dundee, Murdoch, Morton, Eaton, Backlund, Martel etc.