This is a treat. I will begin by saying that I am probably less of a Firelands Portal fan than most players, simply due to its inconsistency. I would pick Flamestrike here; board clears are always a useful addition, and it's as good as they get. It is, however, very close with Firelands Portal, and both picks would be correct. Polymorph can be considered, as there is a large amount of legendaries in Arena, and Paladin players almost always have a Spikeridged Steed. It does not compare to the other options, though.
1. Flamestrike, Firelands Portal