The obvious one is Sinatraa so that doesn't really count, but it's crazy that Dicey nor Babybay have gotten the chance to. Moose stands out as one of the most consistent guys to never do it, and peak Wardell would've been a treat to watch on a LAN. Hazed and EEIU from NRG both also seem like guys who would've been fun to see but never got the chance. And though neither probably are quite at the level to say the "best", Genghsta and Corey both are veterans who have been around so long it seems like they could've
Cloud is for sure the best EU player to never make it, Molsi also has a case as does Jady. Boo is similar to hazed as he seems like the best IGL to never make it from EU, and guys like 7ssk7, f1tinho and twisten also deserve a shout.
I know less about the minor regions but txddy and deryeon stand out as long time players who never made one. RGL is prob the best BR player atm who hasn't made it and Skrossi is the most notable APAC player for sure, and he has an argument for the best to not make it alongside Seoldam.