what is the best small block and why ?

I'm soooo with Mad Dart on this. BUT, also, what is it your going to do with it? Race? Theres some good plus sides to a 340 in stock stroke or stroker form.
If your going to be on the street, I'd go with (And allways do) a 360. Heres why;

Cheaper pistons for many applications
More torque
VERY similar HP and superior torque lines on otherwise identical parts useage.
The HP and torque come in earlier due to the longer stroke.

But it will also fall like a brick after peak HP. Not so for the 340. The HP lines will hold on alot long than what is probably sane for street driving/engine set ups.

No matter what you choose, enjoy! Because that is what it is all about.