what is the best small block and why ?

I vote 5.9 Magnum.

:)

I forgot the why...

They are readily available in ready to use condition, and are 360 cubic inches.

They have a lighter weight rotating assembly than the 340 or 360.

They are hydraulic roller from the get-go.

They seal better and are more consistently machined from the factory than the LA stuff.

The pistons are typically only in the whole .050 from the factory so you have a prayer of getting decent compression with a stock block and rotating assembly.

The heads do crack, but there are better than stock replacements available for less than the cost of going through a set of stock heads (and can be drilled for LA bolt pattern intakes).