It's the least I could do.
It's the least I could have done.
better in response to "Thank you"?
I have seen people using "it's the least I could do" but personally, I feel "it's the least I could have done" is better.
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It was the least I could do
concisely and grammatically expresses the intended meaning, but also grammatical is
It is the least I could have done
The sequence of tenses in
It is the least I could do
might annoy a grammatical purist, but, with such formulaic and phatic language, no one is likely to notice, and anyone who does notice is very unlikely to care.