Countryside around Hastings is very nice and the old town is ok. Much better place than it used to be, suffers from being out on a limb with not many great employment oppotunities.
Bognor has some nice parts like Aldwick and Felpham, Peacehaven would get my vote as worse.
Can't say theres a best it depends on what you want and what time of your life you are in. Sussex is all good.
Littlehampton gets a bad press but the beaches and river are great and great for a young...