Garlic is typically described as having a pungent spicy flavour.

For a single word, I think I'd use "pungent." I'm _mostly_ joking but the second-best word I'd use is "garlicky." Scientists reserve the word "taste" for things sensed by the tongue. In that strict sense, there are usually said to be only four or five true "tastes:" --sweet, salty, sour, better, and perhaps "umami," the taste of glutamate. In ordinary usage, of course, the "taste" of food is a sensation produced by combination of taste and smell. Smells and odors are complex and multidimensional. There are hundreds of words to describe them, and they can't actually be described accurately. In the strict sense, garlic probably doesn't have much taste. Onion does not. If you hold your nose and close your eyes, and someone hands you pieces of apple and onion to taste, it is almost impossible to tell the difference. Both of them have a crunchy texture and a mildly sweet taste. I don't care to try this with garlic, but I think garlic would be the same. To describe the "taste" of garlic one has to go beyond sweet/salty/sour/bitter/umami and use smell-related words. Garlic is pungent, sharp, spicy, and strong. The trouble is that these words also black pepper, onions, or jalapeno peppers. By the way, for me, personally, the "taste" of garlic also includes a sensation of warm, pleasant comfort in the stomach. I don't know any word that describes that sensation.

Bitter is possible, but maybe "acrid" is closer to what you want. At the very least, you could say "sharp".

http://adventuresinspice.com/flavormap/flavormap.html#garlic suggests: "Distinctive, pungent, warm, aromatic, sweet and spicy at the same time." "Distinctive" is quite true, but doesn't help much!