what is the best small block and why ?

To answer the question the way you posed, the 360. Why? More cubes and cheaper to build in stock stroke form. However, to answer the question custom taylored to YOUR specific situation I say the easiest thing to use is the engine already there. The 318. You can always sawp on the bolt ons to a 360 late on down the road.....that is if you're not satisfied with the 318 on the street.....and I bet you would be if you build it correctly. But as stated, anybody can make a shitbox out of anything if they're stupid enough.