I am having a discussion with someone who claims they had a mediocre military that lost more than it won, even at its peak. This seems unlikely to me and is inconsistent with the Wikipedia article on Sparta.
Is there a consensus among historians on whether Sparta’s military was especially good, at its peak?
In its specific niche? Eh, kind of. More can always be said, so if anyone would like to address Sparta and its military, please don't let this post stop you!
For the meantime, OP, I am pleased to inform you that we have an actual, capital-H Historian who's Classical Greek warfare and can give you an expert opinion. I commend to your attention u/Iphikrates' post on whether Sparta deserves its reputation. I also recommend his flair profile, which has a whole section on Sparta.