Hmm. I'll list them...
Aarhus - really liked the atmosphere of this place, with lovely people and tivoli friheden and its beachside location
Birmingham - horrible grey hellscape that I wanted nothing to do with, used exclusively for transportation reasons
Bruges - used for transport BUT I had a wander around this place while we had some random extra time at night and wow this is a lovely place with its small alleyways and huge old buildings that look incredible
Carlisle - only went in the city for transport reasons, but the Center Parcs that I went to nearby was really nice
Chichester - used for transport
Exeter - didn't really like it, got a wrong bus coming back from Crealy & the walk back to the station wasn't fun at all
Gloucester - used this place to climb to the top of the Tor with my dog, great views, quite scenic
Gothenburg - liseberg & the trams are awesome but the actual city itself is a little industrial & grey, but the bike lanes are a huge highlight
London - stuffy, not enough nature, too manmade, no staying power in it
Newcastle-upon-Tyne - really enjoyed the metro for some reason and South Shields was quite nice
Oslo - used for transport
Paris - quite nice actually with very nice staff, a clean metro & the jardins were great. Eiffel tower and Louvre were cool too
Portsmouth - has potential to be great but way overdressed for the weather and didn't manage to enjoy it as much as I should have
Southampton - used for transport
Southend-on-Sea - my childhood dream was to come here as much as possible but my opinion of the place has slightly gone down as I grow up, mostly due to falling out of love with adventure island
St Albans - used to see my friends in college, seemed like a nice place and the birthplace of enter shikari
Utrecht - used for transport
York - used for transport
If I had to pick a favourite I would go for one of these.
Aarhus
Bruges
Gothenburg
Oslo
Paris
Portsmouth
Bonus: