In my observation, the word "good" is mispronounced by ESL students more than any other word in the English language. That's probably because:
It's a common word, so we all hear and utter it frequently.
It looks as if it ought to rhyme with "food."
When you pronounce it as if it does rhyme with food, most native English speakers will still understand you, even though you will be immediately marked as a foreign speaker.
In fact, even among very advanced English learners, the pronunciation of "OO" words continues to be a problem. I know a great many enthusiastic non-native English speakers who continue to sound like beginning students every time they say "cook" or "book" (or "cookbook"), even though they are also comfortable with advanced slang, and watch American TV shows without subtitles.
Why? Well, it's probably because, for most teachers of beginner-level students, the nuances of OO words are too complex to be bothered with. It seems to be enough simply to say that "OO" is pronounced in such a way that it rhymes with the letter U in "Luke."
Quite often, it does. And there are very few cases in which pronouncing OO that way, when it happens to be incorrect, will cause you to be misunderstood. So many English learners simply decide not to worry about it.
But I'm here to tell you that this mistake actually does matter. The majority of my students aren't just learning so that they can read (although that's a great motive too). They're learning English because they want to improve their careers. They want to be taken more seriously by their native-English-speaking colleagues, and they want to improve their prospects with international organizations.
In terms of making yourself sound truly comfortable with English, getting OO right will put you ahead of over 90% of non-native speakers.