Try Premier we stop at these quite often,or the Holiday inn Dartdford which mean you are only about an hour away.
Dover is a dump - I would try and stay in Canterbury for a nicer experience. But if it has to be Dover then Eastlee guest house, Amanda's bed and breakfast or my mum stayed at the Ramada but that was a way out of town.
Premier Inns, as already suggested are usually reliable, but the one in Dover is right on the busy dual carriageway near the ferry terminal and tends to be noisy. The Churchill hotel on the seafront promenade is definitely not recommended as I stayed there on business and it was truly awful. Quieter and better choices away from Dover town centre.