Avoid olde timey candy flavors like horehound. There’s often a reason people don’t eat them anymore. Some flavors are weak and require a lot of liquid, some are extremely strong. Follow recipes. The maker can matter a lot. In most recipes I’ve seen the maker is named with a two letter initial. FW is flavor west for example. Also NO OILS! Not all flavorings are safe. Paper chromatography can reveal the content of a flavor. Butter and cinnamon flavors are particularly suspect. Cinnamon especially as it is nearly impossible to do without cinnamon oil. Diacetyl is similar, but for butter. Clove oil, cinnamon oil, and diacetyl are all bad. You will notice there are no de ent clove cigarette flavors. This is why. There used to be. There used to be really tasty butterfinger flavor juices too. But people died. I was a fan of Kretek and cinnamon red hots. I lived. Probably because I never vaped a ton of either.
Diacetyl is dangerous to breathe but fine to eat. So it needs to be watched out for. “Oil free” flavors and flavors made specifically for vaping are needed.
A note on sour.
“Sour” is usually citric or ascetic acid (vitamin C) which are fine. The powder on atomic warhead candies is powdered citric acid. Lick a vitamin c tablet some time. If that’s the flavor you like it’s the flavor you like.