I've been to every city, town and most substantial suburbs in England, Wales and Scotland (up to Inverness), most of them many times plus Eire once. With cities, I guess my home one (Norwich) might be high on the list. Otherwise, I'd be hard-pressed to choose. All my visits over 34 years were to retail conurbations, so tiny market towns may have been missed.
From my holidays in my Saundersfoot flat, I think Tenby in 'Little England beyond Wales' and its environs make for a nice sojourn. Of course, Bath, Harrogate, York, Chester and the like are good tourist venues. I would include Canterbury as I went to college there but don't think it quite makes the grade.
Of course, you can mentally travel round the coast and find some pleasant seaside places like Whitby, Eastbourne and Torquay, for example.
Great country; pity about the weather though !