A: What do you think of his speech at the meeting?

B: I think  it is/was very interesting.

First of all, I agree with all the teachers who have given their answers to the above question.

The prepositional phrase "at the meeting"which is used as a postpositive attributive  is the key to give a response to the above question. Put it another way, the final answer will be determine by the prepositional phrase "at the meeting"which gives definite meaning in the sentence.

The final response to the above question will depend on the definite and specific meaning that " at the meeting"really conveys.

Both answers are right.

The reasons why I say "both are right" are as follows:

(1)"It is considered to be right that the blank is filled with "it is very interesting "provided/providing that the meeting mentioned above is still progressing.

(2)It is also thought to be right that the blank is filled with "it is was" on the condition that the meeting mentioned above has already came to an end.