So far I most enjoy Tortos, while Horridon is worst so far for me. Considering I'm 4/12, I don't have much experience to draw from so far, so this opinion will almost certainly change.
That said, even though I say Horridon is 'worst', I still enjoy it. Blizzard have kept going strong since T14, which I'm very pleased about. There's no stand-out awful fight in MoP so far.