PHYSICALLY...Luffy is a rubber man! He takes cannon balls to the chest for god sakes. You can't say Ace's strength is inferior to Luffy's because he can't take punches.
Oh please. Blackbeard is said to be a man whose strength rivals that of Whitebeard's! And Shanks did not even know that Blackbeard possesses the Darkness Fruit yet! Luffy is by no means the world's strongest. His rubber body allows him to take greater punishment than possible by human being standard(why am I using human being standard at all for One Piece?). Ace took on several direct hits to vital spots(neck, stomach, heart) from Blackbeard at point blank range. By my personal feeling, the animators could have done the Ace V.S. Blackbeard with a little more flare, like sending Ace crushing into the rubbles and blast a giant 10 feet hole with the momentum. Oh well. In any case, Ace isn't weaker than Luffy by physical standards, it's just that Blackbeard's strength is greater than what Ace can handle.
Ace probably just forgot how it felt to be hit since it's been so long since anyone has laid a hand on him.
And as far as the attacks on Ace go, consider the fight with Enel. He didn't actually take the (Everybody's except Luffy's) attacks in the way that they launched him backwards. instead they usually pierced him but had no effect(and most from point blank range). This is because the fruit made Enel completely composed of electricity in the same way that Ace is completely composed of fire. you can cut of push a part of the fire away but it will still come back unaffected.
but to the guy that said that Luffy was stronger than Ace I kind of agree but i think you still have to consider the fact that not only was Ace complacent with not really physical fighting but when BB ripped out Ace's devil powers it was like having an organ pulled out which is something Luffy has never endured and if even a One Piece universe person did experience this before, and survive, they were probably not so coherent right afterwards enough to fight at their physical best.