Is there a reliable profile of that Tuesday climb in Turkey? If it's indeed 20 km over 10% average it would indeed be tougher than the Zoncolan.
I think it was 18 kms but there is general consensus that it is the hardest climb ever to have featured in a UCI race.
It moves in immediately to the #1 spot on PRC's
page tracking the hardest climbs used in racing, with the RC Cantabria profile they used clocking in at 688 on the Coeficiente APM; including the false flat into it upped the coefficient to 713.
While the Coeficiente APM is a flawed metric - its weighted nature means that it over-values steepness and above a certain level a small increase in steepness can see a significant leap in the score, and it is a pure length/gradient coefficient that has no means by which to account for altitude or surface (although users sometimes apply a modifier to account for these themselves to give some unofficial scores), it does give us a pretty solid foundation to judge.
The second highest coefficient score is 623 for the Alto de Letras, which is of course over 80km in length and also has altitude over 3000m to consider; the third is 594 for Mount Washington, which has its own hillclimb event which is why they included it on the list although that has not featured in a UCI race.
With regards to climbs we should be familiar with from the World Tour, the highest is Monte Zoncolan; they state that using RCS' official profile (beginning at the 'welcome to hell' sign) its coefficient is 543, but using fan-surveyed profiles including some of the lead-in this is around 560 (556, 558 and 562 from the three alternative profiles they reference). Angliru is at 514 using their own data (but this actually reduces with some other unofficial profiles - suspect that this variation may simply reflect different lines taken through corners at some points, noting the hairpin at the start of the Cueña de los Cabres in particular where the inside line is markedly steeper than the outside). Punta Veleno, used once in the Giro del Trentino in 2012, is considered harder than Angliru by this metric but slightly easier than Zoncolan.
Likewise minor variations and interpretation can affect a few others.
Monte Crostis, had it been climbed in 2011 as planned, would have gone right up here with a score of 498.
Großglockner is 496 to Hochtor, but if you put the summit at Edelweissspitze this climbs to 505. However, as the Österreichrundfahrt usually uses it as a pass, this higher score is not really relevant; 460 of these are to Fuscher Torl, the race often categorises Hochtor and Fuscher Torl separately.
Gamoniteiru is considered 492 from its hardest, Pola de Lena side. This makes it considered slightly easier than Angliru as a result despite its superior length.
Finestre is 487 using the official RCS profile but would climb up to 500 including preceding false flat, so would vault Gamoniteiru and Großglockner in that case. However, as mentioned, the coefficient does not weight the scale for surface, so Finestre should perhaps climb above these a little further regardless.
Mortirolo has of course many sides; the Mazzo side is considered the hardest on the Coeficiente APM with 486, with the Tovo side from the 2012 Giro being 463.
Rettenbachferner - which I have long compared to Mortirolo - is slightly easier at 472, but again, the coefficient does not account for altitude, and the fact this is one of the highest points raced in Europe at 2670m is another factor to consider.
Galibier is right up here - but only if you consider Télégraphe to be part of the same ascent. A few here are one-offs or new discoveries like Lussari, or long-forgotten like Kaunertal (not raced since the 1999 Österreichrundfahrt) or Rombo (not since the 1988 Giro save for a one-off one-day race a few years ago), but of regular/legendary ones (or new ones of enough repute), Stelvio from Prato is 446, the Italian side of Agnello is 445, Col de la Loze is 421 from the 2020 side and 435 from the 2023 one; Manghen is 408; Mont Ventoux from Bédoin is 407, Ancares 406, Kitzbüheler Horn 393 (this would climb to 557 if they went all the way to the summit instead of stopping at Alpenhaus); Tre Cime (including Tre Croci) is 389; Blockhaus' hardest version is 383; Larrau is 371; Col de la Madeleine from La Chambre is 366 (and 350 from the north); Gavia south is 348; Mont du Chat is 346; Stelvio from Bormio is 338 (as is Malbun); Kronplatz is 334 (but doesn't account for the sterrato); Alto da Torre from Seia is 332, and from Covilhã is 337; Bonette north is 337 also; Tourmalet is 329; Fedaia (FEDAIAAAAAAA!!!!!) is 327; Tourmalet west is 318 and east is 329; Plateau de Beille is 313 by the largest profile metric; Klausenpass is 309; Puy de Dôme is 302; Port de Pailhères is 301.
Alpe d'Huez scores a 282, for the record. PRC like to use either 240 or 250 as a cutoff for counting a climb as HC, but as you can see how far some of these climbs exceed that figure, it's a bit of a misnomer due to the way above a certain level steepness has such a disproportionate effect.
What interested me a lot however was some of the climbs from outside of the WT in unexpected places. Naturally we expect to see a lot of Colombian type climbs in there - and yes, Letras, La Linea, Salvador, El Picacho, El Vino etc. are there to see - but after Letras, the most difficult climb to be used outside of Europe in a UCI race is in fact the Cerro de la Muerte, in the Vuelta a Costa Rica, scoring 486 from Pérez Zeledón and 447 from San José. This is built primarily on a combo of length and difficulty as it is over 40km and climbs over 2500m.
The famous Ijen Crater climb from Indonesia, home to Peter Pouly destroying the Tabriz Petrochemical Express, scores a 482 for its 12km at 9,6%; Powder Mountain from the Tour of Utah scores a 419 for the version used in 2019 and won by Sepp Kuss, a man so wonderful he even lets other people sit in the front seat of the car, and a 364 for the shorter version used in 2014. Perbatasan Jatim, from the 2018 Tour of Indonesia, scores 397 (its stats look very Ventoux-like), and the long-running Mount Fuji hillclimb from the Tour of Japan is at 391. Sierra de los Comechingones, from the Tour de San Luís when Dani Díaz and Álex Diniz took it to the Quintanas, is approximated at 350 (the APM guys are lacking a detailed profile); Jabal al-Akhdar from the Tour of Oman scores a 317 (although it would be a lot higher if they climbed all the way to the top, but that is perhaps a bit excessive for a February race).
There are a couple of other candidates that I think would be significantly above the 300 score and possibly even 400; the climb to San Fernando via Esquipulas de Palo Gordo has been used in the Vuelta a Guatemala and I estimate at 25km at 7,5%, although nowadays they prefer to ascend this kind of distance into Quetzaltenango via Zunil, a much more classic 5-6% climb for longer; and the
Aynaloo mountaintop finish used in the 2015 Tour of Iran-Azerbaijan, which the official profile suggests to be 24,4km at 7,6%. And, of course, there's also Genting Highlands, an annual fixture in the Tour de Langkawi, which has been climbed from a two-stepped side that cuts its categorised part in half à la Télégraphe-Galibier lately, but the traditional, unbroken side is 20,8km at 7,4% which would definitely fit in this list somewhere.