Darn near everything is subjective on some level. But I have to say, by any objective acting standard, Brad Pitt has proven himself to be a good actor.
There was a time he was average, and capable of a decent performance now and then, but he has gotten much, much better over the years. SE7EN is the turning point, in my mind. He was still playing himself to a point, but he was starting to learn subtleties. SEVEN YEARS IN TIBET helped him in that regard too, and I really think MEET JOE BLACK is one of the best romantic dramas out there because of his performance (and Hopkins, of course). He is fantastic in it. He used his charismatic side to perfection in FIGHT CLUB and OCEANS ELEVEN, and has shown a pretty good range throughout his career. Love SPY GAME, where Pitt, who is essentially mini-Redford, and Redford, had fantastic chemistry together. Even in TROY, which was not the best written or executed film, he gave a pretty good physical performance, whether you think he was miscast or not. He was overshadowed in JESSE JAMES, but he was still very good in it, and BENJAMIN BUTTON was a leap forward in almost every sense.
All you have to do is look at the types of films he is asked to be in to see that the industry considers him a good actor as well. This is someone who hasn't been remotely typecast, and who's been offered a range of films. Remember, he was the original choice for THE FOUNTAIN as well. He's not just a box office draw, he's a talented actor.
The best ever? No. But he's consistently solid these days, and capable of being very, very good just as much as someone like Tom Hanks, Nicholson or Freeman are capable of being average. I think some people just don't like him. Which is ok, as this thread is "actors you just can't stand".
Nicholas Cage is capable of being a good actor, but he doesn't have a lot of range, and has to have roles cast around him. He's a movie star first and foremost these days. He picks big movies to star in, but not good ones.