Scouting Report: Clearly a ball low and away, with poor velocity as well. However, Horford was also aggressively defended by a small child (his one-year-old son, Ean) and was unable to fully step into the throw.
Conclusion: In a vacuum, Crowder's pitch was clearly the best, but we have to take into account the gravity of the situation--he took the field in Double-A, while Thomas and Horford had to perform in the big leagues. The added pressure of appearing for the first time in Boston in front of a new fan base as the Celtics' major offseason acquisition gives Horford's effort the greatest degree of difficulty by far. Considering he was also fouled on the play, the fact that Horford even got the ball to home plate on the fly is quite impressive.
Here's a video recap of Horford's night at Fenway via Celtics.com: