I'm a serious Clydesdale with the climbing prowess of an anchor. Asking about my favorite "walls" is like asking a Hatfield about his or her favorite McCoy. With that disclaimer, here goes.

Toughest ones I've actually been able to pedal my ample self up (not always without stopping to barf up a lung) at some point in my life:

Top o' Diablo. No explanation needed, although DiabloScott's chart justifies all horror stories about it. That wall would be hellish even without the previous 10 miles. But ain't that descent a joy?

Hicks - and that was just to the bottom of Umunumunumunumunum.

Oakville Grade - the first mile of up a little west of Hwy 29. Kind of a stair step, except the flat parts of the steps are 14% - it gets worse from there. Not particularly scenic, not that I was in any position to care. Oh, and it's exposed to the sun, too. Just hateful.

But to me, the hardest remains (drum roll) the top of the east side of Patterson Pass. Not as long as Oakville grade (at least not the part I'm talking about - there is a not-inconsequential uphill run to get there, though, unlike Oakville Grade), but similar stair-steppy unpleasantness with stupid steep ramps. But what makes it so awful is your soul-crushing realization that that 13% wall you just surmounted is the Mother Of All False Summits; only when you have crested it, certain that a lovely descent is about to begin, do you get a view of the top-most (and hardest) climb to the real summit. That initial view leaves absolute no doubt about what awaits. You get a minute or two for the dread to build up while you ride to the base, and the wall is in full directly-in-front-of-you-can't-avoid-looking-at-it view the entire time. Mentally, it's just brutal. If you don't know to expect the false summit - and sometimes even if you do - it will suck the joy of living right out of you, and make you want to commit a violent felony on whoever's stupid idea it was to go this way, even if that person was you. Maybe especially if that person was you. What you don't see is the wind that often whistles through the pass and smacks you square in the face at some point during the final assault. The actual descent into Livermore partly makes up for this foolishness - but only partly.