Which/Who is/are

In both cases use "is".

If there are a few things on the table, which of the following verbs should I use?

What is/are on the table?

If I know that there are several people knocking on the door, which of the following verbs should I use?

Who is/are at the door?

The normal neutral, as Contrex has pointed out, is to use 'is'.

For 'are',

What things are on the table?

What people are at the door? [not a likely use at all but I suppose a situation could arise where it might be used.]

in both cases you would use "is"
otherwise you would have to say
"What things are on the table."
And, "Which people are at the door."