Who is the Most Powerful Warrior in the Tekken Universe?

Well, for me it is always Jin Kazama/Devil Jin!

As for his feats - He has defeated True Ogre, Kazuya Mishima/Devil, Heihachi Mishima, Jinpachi Mishima, and Azazel who would all be contenders for the strongest character as well. The Mishima/Kazama family are pretty much the top dogs in the Tekken universe but excluding them the strongest individual would most likely be Paul Phoenix since he was once a rival to Kazuya and he defeated Ancient Ogre in Tekken 3.

Practically, Jin Kazama has defeated almost every character and is the main protagonist of the series. As such, he has appeared in the most prominent spot on the cover of the games almost every time.