I'd bet on Paulie. He was so pissed at Ralph for the prank on Paulie's mother and was still pissed with Tony. Although such a move might be too "smart" for Paulie, still he's my ultimate suspect if this fire was not accidental.

Paulie definitely knew T. to be fond of the horse and knew that Ralph was too much of an a$$hole to care for such a thing; therefore, it'd make sense to make such a move that was so horrible that only a fxxxed up person like ralph was capable of and hit Tony where it hurts. He had been "financially" hurting Tony but nothing made Tony feel as bad as an emotional hit ("...when they come, they come at what you love..." Mike in III) while at the same time blaming it on Ralph. Except he didn't (and nobody could) foresee the accident to Justin and that Ralph couldn't logically be in the presence of mind or emotions to plan such a move. I think that an un-emotional Tony who wasn't "clouded by his anger" (although he mentioned he controlled his anger an episode or two prior to that) would have ruled that it wasn't Ralph who set up the fire.

Besides, Paulie has to go in season 5. If for nothing else, because he opened his mouth to Johnny Sack and Tony will eventually find out. This is another reason to add to Paulie's demise. If he could get off with the money he cost Tony, he won't be let off the hook with the grief he caused the Boss.

A point to add is the painting he kept after Tony asked for it to be destroyed. Paulie might end up showing it to Tony to try and show off his "loyalty" that he kept something that "glorifies" the Boss.