Big county, big question. If the sun if shining, then any part of Sussex is nice. Personally I like anywhere near the South Downs. My own highlights and lowlights are as follows:
Very nice - Rye, Chichester, Horsham (I live there!), Arundel
Quaint - Eastbourne, East Grinstead, Petworth, Steyning
OK - Lewes, Haywards Heath, Midhurst
Too busy - Brighton
Not so nice - Hastings, Littlehampton, Bognor, Crawley, Newhaven
Good sandy beaches - Camber, Littlehampton West Beach, Wittering/Bracklesham Bay
If I had to pick a single place, it would be Chichester. Excellent shops, loads of campsites, cathedral, close to A27, near the Downs, 7 miles from the coast, loads of restaurants etc.
Mike