what is the best small block and why ?

There is NO such thing as best when it comes to this. It is ALL subjective.

Yea, if you were talking almost forty years ago, the 340 was the engine to have.
Today it is more of a "wow, look at me because I have a 340" type thing. If you blow up a 340, then there is another one gone. That said, I am NO WAY slamming the 340. It was a potent package back in the day and they run real strong. The newsflash is that all the stuff that made the 340 run great can be applied to the other small blocks as well. The buy in price for a decent 340 core is more often way to much money. Unless you find one at a decent price, I'd skip it.

If you are of course rebuilding it from the ground up, the 340 vs.360 argument becomes pointless nonsense. The only thing I can see then is higher rpm or more low end torque. What fits YOUR needs better?? What can YOU afford??

The most economical choice is the 360. It offers the most for the least by far if we are talking talking stock stroke engines. Around here in the south, they can be bought cheaply. Pistons are reasonable for them to. They run real strong with minimal modifications. Easy to obtain as well..

Stroking these engines out really levels out the playing field as well. If you want crazy torque, consider a 408.

Another option is the lowly underrespected 318 . Dirt cheap and easy to obtain with the downside being a higher cost for pistons. Get a good set of heads on a 318 with a nice 10:1 or so compression ratio and you have a very strong street package. The combinations are endless and if you want a 390 cubic stroker engine, it won't dissapoint. The 318 shares the same stroke as the 340. The 318 is another budget minded option for those who already have them or can get them cheap. Don't dismiss them as a pig for performance. There are several 318's out there that can surprise you!

Again ,it is not the 70's anymore and quit living the past. I am a nostalgia freak for sure but with today's performance parts, you can make ANY of the Mopar LA engines run DAMN GOOD!!

I am still a believer that the correct combination of parts is the secret to a great running engine rather than it's cubic inches...