* 間抜け (manuke): This is a common and relatively mild term for someone foolish or silly. It's a good general equivalent if you want to convey a lighthearted sense of someone being a bit of a goofball.
* バカ (baka): This means "fool" or "idiot" and is much stronger and more direct. Use this only if you're comfortable with a more insulting tone.
* アホ (aho): Similar to "baka," this is also a strong word for "fool" or "idiot," but it's often used in a more playful or affectionate way, depending on context and tone. It's more common in western Japan.
* 鈍感 (donkan): This means "insensitive" or "obtuse," and could be used if the knucklehead is more characterized by their lack of awareness than their outright foolishness.
* 頭が悪い (atama ga warui): This literally means "head is bad," implying a lack of intelligence. This is quite blunt.
The best choice depends on your relationship with the person you're describing and the level of insult you intend. For most situations, 間抜け (manuke) is a safe and appropriate option.