Here, again, are a few possibilities
ALTO DEL ANGLIRU
It's pretty much unquestioned as the hardest climb in Spain, such that some have thought it's too hard even for seasoned professionals. When the weather turns sour, as it did in the 2002 Vuelta, the climb becomes almost literally undoable.
MONTE ZONCOLAN
Zoncolan will no doubt claim a few souls in the 2010 Giro. It's a name that strikes fear (or, for some, excitement) in the hearts of those who are meant to tackle it.
ALPE D'HUEZ
Be honest - this is the first climb you thought of when you saw the subject line, isn't it? It's a name that's rightly legendary in Tour de France lore, and if you win here, your name automatically goes down in the history books.
COL D'AUBISQUE
This evil climb doesn't quite have the reputation that Alpe d'Huez does, but it probably should. It's been visited nearly as often, and is every bit as much leg-burning torture.
MONTE PETRANO
Petrano was probably over-glamorized just a bit this past Giro, coming as the capper after two other demonic first-category climbs, but a fitting capper it was, with height and gradients to match almost any climb any stage race can offer.
ALTO DE SIERRA NEVADA
You'll be hard pressed to find a higher climb that cyclists will tackle anywhere in the world. The marquee climb of the last Vuelta saw time gaps of nearly 40 minutes and a close to a dozen riders not even try to complete the stage.
COL DU TOURMALET
The Tourmalet might not strike fear in seasoned climbers as much as some of these other climbs, but it's still a doozy. One of the highest climbs in France, frequently the highest point reached in the Tour de France or the Dauphine Libere, it will be visited twice in 2010, leaving those who live to battle gravity licking their lips.
PASSO DI MORTIROLO
Not a climb of epic height, but its steepness has proven selective when it's been used in the past. This climb almost cost Danilo Di Luca the 2007 Giro.
PLATEAU DE BEILLE
What flagrant false advertising, to call this climb a "plateau!" This is the climb where the current practically-undisputed world's best cyclist first showed his true prowess, defeating an artificially enhanced Michael Rasmussen to the finish.
So, just some of several possibilities. What do you think?