Student or Learner
I'll never forget what you did for me. Thank you.
I'll never forget what you've done for me. Thank you.
Colloquially speaking, which of the above makes more sense or more common?
What you did refers to one singular instant of good. You jumped out in front of that bus and pulled me to safety.
What you've done refers to a period of support. You have always been there for me whenever I needed you.