The first episode of the second season was the best. It was just so twisty. The Hounds of Baskerville was hobbled by its fidelity to its intractable source material- and strayed too far (for my taste) into the realms of SF. Wasn't it Dorothy L Sayers who said "poisons unknown to science" are a no-no in detective fiction? The same surely applies to wholly fictitious hallucinogenic gases.
The Reichenbach Fall was almost up to the highest standard. I like Andrew Scott's geek Moriarty, but I do hope he's really dead. Conan Doyle was very sparing with the super-villains- and one of the strengths of the original stories is the way he anchors them in the mundane- and sordid- realities of late Victorian England. National security and the fate of the crowned heads of Europe are very rarely entailed. My plea to Gattis and Moffat for Season Three is, "Rein it in a bit, chaps".