If you're thinking of a city any of the big ones are great. So much to see and do. There's a reason why they're so popular!
Venice is my favourite. The first time I went I was not expecting to love it, thought it would be corny, and hey we've all been to Amsterdam right, so what's the big deal? Omg I couldn't have been more wrong. I've been back three times since, just for short breaks, and can honestly say there is nowhere else like it. It is truly unique. It literally rises up out of the water - they hammered thousands of tree trunks into swamp and built on top of them and when you're there you totally get just how impossible a place it is.
In addition it means that getting around isn't like getting around anywhere else. So there are no roads. There's land you can walk on and then there's water and that's it. If you want to go somewhere you can't walk to you get a boat. It also means that because everyone is either walking or getting boats people just behave differently. There's more space than you think when everything isn't taken up by vehicles and all sorts of corners and nooks and even in the peak season you can turn a corner and walk down a street that is suddenly quiet.
Outside of peak season and actually even within it (summer storms!) weather roils at you unhindered - I mean, you're there, on the flat, surrounded by water, on land that it doesn't make sense to build on. You get it all, separate to the rest of the world.
And then there's the art. Venetians spent a long time invading and plundering everywhere they could, killing each other to get their hands on what they brought back, and stashing it all over the place in a mad mishmash culture frenzy. Plus they grew their own world class painters and put the works they created everywhere. So you'll be walking down the street and see a wee museum sign for a building that charges you two dollars entry, and when you go in it's all fucking gold leafed up to the hilt and stuffed with Tintorettos.
... Anyway. Venice is ace ...