Assuming that all of DC comics Superman's powers have completely and totally matured, which one of them was the last power to fully develop? I haven't read each and every universe, but I'm pretty certain he still has his long established set in his arsenal.
Superman has a history of sometimes developing different powers at different times. This was extremely prevalent back in the Golden/Silver Age when they needed a new super power to handle a new super threat.
They toned this down more so in the modern age, however just this year Superman developed yet another superpower. Here's an article detailing this new ability.
So, to more specifically answer your question, Superman's powers seem to be in a constant state of flux depending on the writer. Just when you think the Man of Steel can't get any more super, suddenly he's got a new trick up his cape.
Just as @splitting-adam said, This article outlines his X-ray/ heat vision as being his final development. This is a prevalent trait throughout his mature life as well. The comics detail his murky 'double vision' occurrences during his early years, but never exactly explains the heat ray until after other powers are developed or explained.
Although it's not exactly a "power", after every super power is explained, his allergy to kryptonite is described. This is an example of a 'super power' because humans can not exhibit the same reaction, therefor being a 'super' or 'superior' power. Even though this is a downfall to the character's strengths, the description of this weakness is his last described power/feature.
UPDATE: This article explains that his "telescopic and microscopic vision" was one of the last to develop but it depends on the actual time in history you are looking at. And that the comics developed depending on the author.