When it comes to snacks, the catering in each place has its own characteristics, especially the places you mentioned have obvious styles, depending on your taste. Hangzhou is light, Chengdu-Chongqing is spicy and delicious, and Xi 'an is honest (Chinese hamburger+cold rice noodles set meal, staple food with staple food). I suggest you search for delicious food from all over the world, and then compare it yourself before making a decision.
Personally, I think these places are worth visiting as long as there is time. Just go down one by one, the important thing is to start now ~