I wouldn't say "we're always best friends." "We are" infers that you are *currently* best friends, whereas "always" infers that you *have been* friends from the beginning. Putting it together almost says "we are currently always", which is a bit confused. It would be better to say, "We've always been best friends", which is then very similar to, "We have been best friends for a long time."

What about "We're BFF"...? :)