This is not of great exegetical value, but thanks to a discussion on the Accordance forums, I can report that there are 7 one-word sentences in the Greek NT (+ 1 in the short ending of Mark).
Matt 11:9 προφήτην;
Mark 4:3 Ἀκούετε.
Luke 7:26 προφήτην;
Acts 15:29 Ἔρρωσθε.
Rom 3:9 προεχόμεθα;
Rom 3:27 ἐξεκλείσθη.
2 Pet 3:18 ἀμήν. (There is a similar instance of ἀμήν in the short ending of Mark.)
If we get down to the number of letters, that makes 2 Peter 3:18 the verse that contains the shortest sentence.
This gives you sentences as defined by some editor’s punctuation, but may not all actually be complete/independent sentences (e.g. Mark 4.3; but also Matt 11.9 // Luke 7.26 and other rhetorical questions within a dialogue; e.g. Rom 3.9). Also if 2 Pet 3.18 is a sentence, then there ought to be others as well (e.g. Rom 9.5; 11.36 etc.).
What is a sentence anyway?
All very true. As I said, “This is not of great exegetical value.”