I'll second the pelican. Not just for ar15s but anything you actually want, long term. I have a bunch of pelican cases and while they are expensive and heavy, they last for decades in real use.
On the soft side my only advice is to avoid airsofter junk. There are a bunch of clonesoft cases that are worst of all worlds... expensive ($100+), cheaply made (snagging zippers, untaped seams, etc.), and broadcasting ".mil wanabe" on all frequencies. You are better off with a $35 Allen or the like.