There's no real good answer to that, because most manufacturers have released their share of good and bad kits over the years. To say Brand "A" is "better" than Brand "B" would be an extreme oversimplification.
If I can make one very general remark, I'd say that overall, over the years, Japanese kits have been generally better designed (crisper molding, better engraving, more precise parts fit, etc.) than American brand kits...but that's not always the case! Also, be aware that many Japanese kits are/were issued as "curbsides", meaning no opening hood and no engine! See how trying to make a blanket statement as to which brand is "better" than another can get messy?
If you're looking to build a model of a specific car, for example, your best bet would be to ask about that specific car/kit here on the forum. Who makes the best model of a (insert car here)? Who's kit has the best detail? Who's built that model and has any insights?, etc. You'll get more useful information asking specific questions like that rather than a generic "Which brand is best" type of question.