This is one of those questions that begs: Well, how are you measuring, "best player"?
Any answer is good, if the player you name represents what you think is best. Yeah, that's a lame answer, I know.
For the sake of something to talk about, turning to dry, cold statistical measures like Win Shares, I expect it to be close between Isaiah and Horford.
Horford stayed healthy for 82 games and logged a fantastic 9.4 WS last season at a .172/48 rate. That's his 3rd highest WS total of his career and a rate that is solidly above his career average, so I'm not expecting him to decline much, if any from those numbers. At this point in his career, and given his natural fit into what the Celtics try to do, I would not be surprised if he did those same numbers again or even slightly above them. I also note that Amir, in lesser playing time, also turned in great WS numbers and so I think that bodes really well for Al. I'm going to predict he turns in 10.0 WS even at about a .182 rate, which would be close to but just under career highs. Those numbers are not calculated, just educated guesses and of course assume he stays healthy and plays the whole season.
Isaiah also played 82 games and just squeaked out a higher 9.7 WS, at a .177/48 rate. So you can see how close they both were in value to their teams last year. Now, Isaiah is much younger than Al and it's not unreasonable to think he still have a fair bit more upside. Especially if the lineup around him improves our spacing. I'm going to guess that, again, assuming a full healthy season, he could post a 10.5 WS season at something like a .185 rate. If anything that may be a bit conservative based on his trends, but I'm also expecting his USG to not be quite as high.
So I'm going to give the nod to Isaiah, but just barely.
And of course, all this means is that somebody we haven't thought of will have a break out year!