Sorta skipped around on this thread. Some funny things I found is whenever Core is listed it's described as, "pretty good." I don't know if you can have an okay disc as underrated. Shark is a great disc... but then why not just throw a Buzzz... you know, because it's a better version of the same thing? Coyote just reminds me of JR on DGR which he raved about constantly so how can that be underrated? Also because of DGR, Comet cannot be underrated.
Starfires and Orcs are good discs but the PD is what they wanted to be. Surge, Rogue and Avenger are other discs the PD beat out. (Never thrown a Viking but assume it can be added here. Valkyrie is appropriately rated, IMO.)
Rhyno another good disc but also popular so the definition of underrated is disputable. Same with Aero and XD.
Aviar and DX Teebird are probably the best selling discs of all time. Again, not underrated.
Impact and Glide two discs I've never thrown or seen anybody throw that merits further research. Same with Viking and Monarch. Also any Gateway disc that isn't a putter.
Anyway, my vote? Cyclone. Admittedly a tweener disc that I don't bag...
My other vote? XL. Unfortunately it molded up best in OOP Elite Pro plastic. Actually so did the Cyclone....
My other, other vote? Fuse. Okay, I'm done.
Also, Mako is probably the best beginner disc.