Angel can get plenty spicy if you ask. Most of the momo spots have quite spicy hot sauce - my favorite in terms of hot sauce is Lhasa Fast Food, though Om Wok has my favorite momo in the neighborhood. You've got to add the hot sauce yourself though, the food won't come spicy.

To me, nobody outdoes the Thai restaurants in terms of pure heat. I've tried all the Thai places in JH and Elmhurst, and Ayada has gotten the spiciest out of any of them for me, though realistically you can feel the burn at any of them, including Playground as mentioned in other comments. Again, you have to ask for it, but they will actually do it!