Echoing most others, I adore Stockholm and Copenhagen. Neither are perhaps as full of interesting 'things' to the extent of some of the big(ger) hitters, but the vibe in them (and Oslo, but fuck me at trying to eat and drink there) is just so...I don't even know, so magical. Just feels like the places you wish you had grown up in as everything and everyone is so well put together.
Oddly I visit Spain the most but it's mainly due to the weather and price, however Seville as a city I loved and Valencia would be perfect for a weekend.
Berlin is obviously amazing, Munich pretty fabulous too.
Budapest I adored and for weekend getaways (i.e. enough to do to fill your time in) places like Porto, Warsaw (per above) and Dublin I've found great for 2-3 day trips.
Overall there isn't too many that I've found a bit shit. Sure, some are much better than others, but if you can't find enough to fill in a city break then it's a bit baffling. Still, I'm a complete sucker for just spending a day wondering and just getting a vibe from a place, nibbling on the food, having some wines etc.