Vollering is the on-paper favourite, she has been the season-long strongest climber over the last few years. PFP is a wildcard because she's been out of the road péloton for so long and she hasn't done much of this type of climbing, if any at all, since her return (she was out of the Vuelta before the mountains began, and came 18th, three and a half minutes down, on Jebel Hafeet in February) - but PFP mark 1 would have won the Giro in 2014 if not for bonus seconds, albeit there were no climbs comparable to the Madeleine in the Giro in her heyday (she didn't do the race in 2016 when the Mortirolo came in). You'd say given the proven record over climbs as tough as this that Vollering is favourite over PFP, but PFP's comeback has essentially been built around this race, so this is her looking to be at peak here.
Van der Breggen is also a wildcard; in her heyday she was the top climber out there, and any tough race became essentially a straight slapfight between her and van Vleuten. She has looked a step off that level since her return, but was still the next best climber after Vollering and Reusser in the Vuelta, and has been targeting this race also. After last year's SD Worx shenanigans, there's a bit of needle between her and Vollering as well.
Niewiadoma is the defending champion so should never be counted out, but at the same time she's a great climber but also somebody who has been beaten by Vollering in pretty much every climb of this scale in recent years (Küssnacht in the Tour de Suisse being the only exception I could find on a quick scan); last year's victory was built on a spectacular defence of a lead she was granted by SD Worx' implosion of teamwork in a flat stage. However, for much of her career her sweet spot was mid-length climbs of 5-8km and she suffered on the longer ones, she has got over that limitation and has been defeated only in the sprint to the line a couple of times, most notably Torgon in the Tour de Romandie a couple of years ago.
Gigante is definitely a potential wildcard, the problem is that while she's long had the engine, she'd only shown glimpses of her max potential until recently, and you can only take people by surprise once. The field in the Giro was perhaps not too much worse at its pinnacle than the Tour, but it was certainly less deep - no Demi, no Kasia, no PFP, and it was clear van der Breggen was not at her best. Reusser has been the best version of herself this season for sure, but she got sick at the end of the race, while Kopecky got the opportunity to lead in order to free up Tour leadership for van der Breggen, and completely flunked the chance. ELB is, however, long term proven quality, plus of course she's Italian. As a pretty one-dimensional rider, Gigante is likely in a similar role to the one she had in the Giro, or Pauliena Rooijakkers had in last year's Tour. Speaking of Rooijakkers, we probably shouldn't discount her from the potential to do the same as Gigante, after all she was up with Vollering until the line last year, and last year she was 4th in the Giro before 3rd in the Tour, and she's been 4th in the Giro again this year. Kerbaol is also an outsider with potential to surprise - she hasn't got a long list of examples as she's really stepped up this season, she was 4th on Cotobello and 6th to Lagunas de Neila in the Vuelta and 4th to Küssnacht in the Tour de Suisse.
Hell, last year Neve Bradbury won on Blockhaus, she's here but hasn't demonstrated the kind of form to suggest this is remotely possible again. And there's always the potential for a one-off showing. Évita Muzic was 3rd to Alpe d'Huez last year despite being caught behind the péloton before the Glandon after a nature break, but she will be riding in the service of Vollering; Marion Bunel has been very inconspicuous in this race but rides at her best on the biggest and baddest climbs, being 11th to Alpe d'Huez last year as a 19-year-old before winning on Les Karellis and the Colle delle Finestre in l'Avenir, and being 2nd only to Vollering on the Coll de Pal (the Bagà one, not the Andorra one) in the Volta a Catalunya. And who knows how far Maëva Squiban's current form can take her?