It has to be Krakow for me, especially in the summer.
It's the spontaneous aura in this place, as well as the fact that it literally has everything but also it's history and overall vibe. So many times i've been there and there's always something going on - people dancing together in the main square, Jared Leto appearing out of no where and giving a concert on the street, Speed and other youtubers showing up, the dragon parade, etc.
It has a giant lake with crystal clear water, that is an amazing cliff diving spot.
And overall a bunch of cool shit, like the big ass helium balloon that takes you up in the sky and gives you a view of the whole city.
Never got bored of it.