How best to respond to a super low-ball offer?
I suggest sellers ignore it and pretend it never existed. Do not entertain or respond.
How to best respond to a lower than desired offer?
I suggest you counter in such a way that keeps the buyer engaged, but you don’t give away the farm on your first move. Buyers rarely make their “highest and best offer” initially.
How to respond to a really great offer?
Sign it. ASAP. Do not try to tempt fate or be greedy. Sign it. It is very rare that such a strong offer can or will be duplicated.