I look at Sengoku in the same way I look at Yamamoto from Bleach. You can just feel that he's incredibly powerful, but the fact that he never fights makes you unable to really prove it. Recently in Bleach, Yamamoto displayed that he is more powerful than at least four vice-captains combined. That being said, I know we're not talking about Yamamoto, we're talking about Sengoku.
But by similar retrospect, Sengoku could, quite possibly, be more powerful than any of the Admirals. However, the reason he is not considered a part of the "Ultimate Fighting Force" is because that, being the Fleet Admiral and the person who has to deal with all of the workings of MHQ as well as the fact that he probably hasn't needed to fight anyone himself in over twenty-thirty years, it's obvious no one would really consider him the strongest marine because they wouldn't really know for sure.
Now with that mini-rant out of the way, here's how I view Sengoku's power:
I believe that his goat is a Zoan user and is, in fact, a decorative katana, probably a White Blade, in contrast in power to Shuusui and Mihawk's Black Sword. That being said, I don't think he posesses any kind of Devil Fruit because he himself feels that such a thing would be more of a hinderance as it would make him weak against kairouseki as well as being reliant on his devil fruit so much that haki users could exploit a weakness in his own technique that would otherwise take him by surprise.
Also, I value his fighting ability just slightly less than Whitebeard's own initial strength. So while Whitebeard could decimate Sengoku in a very brief fight, Sengoku would be victorious if the fight becomes drawn out.