1. Which are more important?

2. Which are most important?
3. Which is more important?
4. Which are most important?
5. Which is the most important?
6. Which are the most important?

(Which one is correct? Are all OK? What is the difference?)

All of those are ok. When you use the word "is" you are talking about one thing.

When you use "are" then you are talking about more than one.

When you use "More" you care comparing two things.

When you use "most" you are comparing more one thing to many other things.

All of the options you listed are grammatically correct and can be used in different contexts. The choice between using "which is" or "which are" depends on whether you are referring to a singular or plural noun.

1. "Which are more important?" is correct when referring to multiple options or items. For example, you could ask this question when comparing two or more factors or criteria.

2. "Which are most important?" is similar to the previous question but expresses a superlative degree of importance. It implies that you are selecting the most important out of a group of options.

3. "Which is more important?" is used when you are comparing two specific items or factors, treating them as singular nouns.

4. "Which are most important?" is similar to the previous question but expressing a superlative degree of importance for multiple items or options.

5. "Which is the most important?" is used when you are comparing multiple options but looking for a single superlative answer. It implies there is only one most important item or factor.

6. "Which are the most important?" is similar to previous question but asking for multiple superlative answers. It implies there are multiple items or factors that are equally important and you want to identify all of them.

The difference between these options is the number of items or factors being compared and whether you are seeking a single or multiple most important ones.