most impressive app about to be killed by iOS: a puzzle game called Blockhouse. Released in 2009, never updated, still works with 0 bugs (that I have found) today. Letterboxed, of course, since it was made for the original screen size.
I'm impressed not because it's all that complicated, just that it has continued working across so many OS revisions and has not needed any updates at all, ever, for any reason. I can't think of any code I've ever typed that was bug free.