Best Years for the 318 LA?

"Best" will vary depending on your goals and the intended build/usage.
Looking for a basic stocker to freshen and bolt into a mellow street cruiser? Late roller block. Or even one of the early '80s cop 318 4 bbl. motors.
Something to poke and stroke and get a little hairy with? Early teener- blocks were thicker and can take a bigger overbore- guys make "stealth 340s" out of 'em. More likely to find the steel cranks in the early ones, if you want to stay stock stroke.
A raspy little street brawler, like Rusty said- any of them will be fine. Main thing is to get the compression up- decent pistons and the early heads (or a good set of 302s if you can find them) will get you a solid foundation. Do you really need a forged crank or massive overbore on the street, or street/strip? Probably not. Just depends where you're going with the build.