Snape was clearly a competent DADA teacher (as opposed to some of the others before him).
Is there any example in canon (books/Pottermore/interviews, or even movies) that show unambiguously one of the things taught by Snape to Harry's 6th year to come in useful to one of Harry's classmates in books 6 and 7?
This excludes anything that DA members are known to have learned in DA from Harry.
You may include items that are not 100% clearly shown to be taught by Snape, but which:
Were not shown to be taught by anyone else earlier
Were clearly something that would be taught in DADA class and not some other class.
Ideally, shown that the person using the skill would NOT know it until DADA with Snape.