Budapest, by a long shot. I've been living here for 6 months, and the place never ceases to amaze me.
Besides the main sights that you'd expect to see in many European capitols (beautiful palaces, opera houses and churches, main shopping streets and squares), there are many surprising (and often hidden) things to see here.
To name a few: more than 5 different Turkish Baths, caves under the Budapest Castle, public libraries that look like Versailles inside, coffee houses that rival the best in Vienna, the first underground/subway built in continental Europe.
Furthermore, since much of the city was built in the late 1800s'/early 1900's, and remains largely intact, the 'old city' atmosphere goes on and on, through many city blocks.
There's also been an amazing amount of development in the short time that I've been here - it's a golden age for cobblestone layers, as they've ripped up many ugly streets, and replaced them with brick. Several old buildings are continuouslly being refurbished as well. It's happening all over Pest.
Check out this site
here, to see what I mean.
Oh, and there's an entire shopping distrct/street called Kiraly Street where there are maybe 8 or 9 major building restoration projects going on. All high-end apartments.