Well, that's the answer: hot basil. That's the only type of basil Thais use to make delicious Krapow. Also called bai krapow, or holy basil, its heavenly smell is most distinct in the nostril-tingling, aromatic spicy whiff in the world-famous stir-fry.
The red-purple stemmed leaves of holy basil leave a peppery, numbing flavour to the Krapow meat and rice one-plate meal. It’s also hot when fresh - that's how it got its name. When stir-fried under fierce heat, as how a Krapow dish is normally made, its juices seep out and flavour everything with strong spicy heat. And the more you cook it, the stronger the flavour builds. That's why it's often eaten with rice to soak up all the addictive juices, and a crisp runny egg on the side to mellow out all the heat.
Now, before you go hunting for the herb, know that they're a part of a big basil family - it's a close cousin of the Thai sweet basil, and not so far off from the Mediterranean basil that's used in pasta and pesto, among others. The hot basil though, has two variants with widespread use in Thai food:
White Hot Basil (Krapow Khao) – broad leaves, with milder, peppery clove notes you can use for seafood. Good to stave off colds, relieve nasal congestion.
Red Hot Basil – broad leaves, with stronger, well-rounded, peppery clove notes that go especially well with meats. Good to stave off colds, relieve nasal congestion.
Others, like Thai sweet basil and lemon basil are also commonly used in Thai cooking:
Thai Sweet Basil – sharper, narrower leaves with sweet, liquorice tasting notes, used to give curries, seafood soups, and salads a sweeter, balmy flavour.
Lemon Basil – soft fuzzy stemmed leaves with lemongrass-lemon-lime notes. It’s an essential ingredient for a delicious Southern style fish curry known as Khanom Jeen. Nam Ya. In the North, its holds equal importance in a pork curry noodle soup, or Khanom Jeen Nam Ngiao.
Outside of Southeast Asia, your local supermarket likely stocks the more common Thai sweet basil. Specialty stores, or online grocers though, might have the hot variant.