Personally I think the best is:
alphabet="abcdefghijklmnopqrstuvwxyz".split('');
Concise, effective, legible, and simple!
EDIT:
I have decided, that since my answer is receiving a fair amount of attention to add the functionality to choose specific ranges of letters.
function to_a(c1 = 'a', c2 = 'z') {
a="abcdefghijklmnopqrstuvwxyz".split('');
return (a.slice(a.indexOf(c1), a.indexOf(c2) + 1));
}
console.log(to_a('b', 'h'));