Uhm... the best ptich in baseball is strike one...Batters can touch any pitch upon occassion...For a good review of tough pitches made by hall of fame pitchers take a look at Ted Williams book "My Turn at Bat."
Williams called the low outside slider "the Bas--rd pitch. I think part of it was because a number of umpires would call it for strikes. That being said the cloest thing to an unhittable pitch was Hoyt Wilhelms 2 strike knuckler. Thjat was the one that he took all the spin off and just let it dance. Wilhelm could actually somewhat control his knuckler by adding a rotation or 2. For instance, to get strike one he would add a couple of rotations of overspin to get it to come in and basicially just drop. Williams said the two strike knuckler was "like a butterfly gone berserk."
Even a spitter thrown high is a homerun ball.
For me a ball that is sharply droping in the strike zone is toughest:
Blylevens-over hand curve
are good examples, Ian.