what year of 360 is best

"what's the best?"
The one that's not damaged and cheapest. I personally don't like Magnum heads for anything more than mild builds, and Magnum heads can go on the LA block. You can't put LAs on a Magnum block easily. So in terms of cores, I look for any pre-Magnum engine. The later blocks are more consistent, but I won't say they are better or thinner, just more consistent. Sometimes consistently bad...lol. Don't waste money on the "thicker '71" blocks because it's a crap shoot that you get one that's not core shifted so much it's no better than the later ones. The later 80s trucks have them and i've gotten lucky enough to score running examples for free so long as I go and pull them myself.