I have a memory of seeing Spock say the line "Impressive, Captain", but now I can't locate where that was. Am I misremembering this?
In a discussion between Spock and Kirk, "Unquestionably. Most impressive." is the closest quote I could find...
There was one instance of that exact phrase in non-canon licensed books, but it wasn't Spock speaking.
Star Trek: Signature Edition: Worlds in Collision By Judith Reeves-Stevens:
Very Impressive, Captain. I would have sworn you had tuned me out completely. (src)
There didn't seem to be any phrase exactly like that in TOS episode transcripts, though it did occur in one TNG episode.
However, there were several quotes that were somewhat similar in nature:
SPOCK: Unquestionably. Most impressive. (Spectre Of The Gun)
SPOCK [OC]: If I may explain, Captain. She refers to the tape storehouse of knowledge of the builders of this place. I scan it. A most impressive store.
Kirk himself said something similar in a couple of episodes.