Chengdu was recently listed as a UNESCO world culinary city. Sichuan food is one of the most popular, if not the most popular, styles of Chinese food, and Chengdu is the capital of that Province. I'd say that would be a good place to start - they're famous for fiery dishes like crazy spicy hot pot, spicy braised fish, and meat stir fried with dried chilis in a 4:1 of chili to meat ratio haha. I'm trying to remember the names of the dishes, but I can't right now. Luckily in China most menus have pictures, so you can pick what looks good to you.

Hong Kong is good for dim sum, char siu, etc. but if you're on the mainland, Guangzhou has equal quality of all that stuff (All Guangdong style food). Guangzhou has amazing late night food as well. If you go to dim sum, get the daikon cake with XO sauce. Although, if you go to proper dim sum and you can't read the menu, finding the good dishes will be hard. A lot of restaurants put little stars next to their popular dishes, making them easier to find, but typically dim sum places don't have a lot of pictures, they give you a card with all the names of the dishes in Chinese and you check off the ones you want. You might have to recruit people around you who have good looking stuff on their tables to show you what the name is on the menu (I've done this without shame on many occasions).

Another good place for food is Yunnan province. The only province in China that makes their own cheese, a style of goat cheese that they stir fry with tomatoes and it's goddamn incredible. If you're in Kunming, go to Heavenly Manna on Wenhua Xiang, one of my fave restaurants in China. English on the menu as well.

For Thailand, unfortunately I don't know a lot of names of dishes, but you must check out eatingthaifood.com. I found some INCREDIBLE restaurants in Chiang Mai on that site. Also, the guy who write for that site has a youtube channel (called Mark Wiens), where he goes to all these street food places and restaurants in Thailand. I'd say it's required viewing if you're going there.