@strictlyanime: Yeah, Aang was successful in stopping the firelord. But the repercussions of each (possible) failure are incomparable to say the least.
If Aang doesn't stop the firelord, sure the war continues, but eventually another avatar (or perhaps another non-avatar bender) would step up to finish the job.
If Wan did not defeat Vaatu, not only would the ENTIRE world plunge into eternal darkness, but Raava would cease to exist. Therefore the avatar would cease to exist. Therefore (insert worst-case scenario here) would happen.
It's nice to give Aang his credit (such a humble avatar) but in all honesty his task was completely overshadowed by Wan's. It's pretty unanimous. Wan created bending; before his style and growth with the spirits ppl used the elements like primitives. He did everything he could to unite the spirit and physical worlds (until he realized it best to seal them off from each other, becoming the sole bridge between them).
If we're comparing capabilities Aang has no experience with spirits whatsoever (whether communicating with or purifying them--something even Korra is learning). He did a lot of good for the physical realm. But Wan did everything for them both.