you'd think with only seven episodes, it would be easy to pick. *sighs* they're all so good (well, haven't watched Fall in it's entirety yet, but still) with such good scripts that it's really, really hard to pick just one.
we'll do it this way. for my favorite pairing, it's The Great Game because there's a lot of Sherlock and Lestrade interaction in that one. the puzzles are great and there's lots and lots of Sherlock exposition, which is always just hot. i swear that Ben uses a lower voice on purpose when he does those sequences. *grins*
overall, it's Scandal in Belgravia. that one has the best script, best character interaction, best lines, best everything. it's the one i watch when i need something to giggle at (how many times did he fall out the window?) and also when i need to be able to get either Sherlock or Mycroft's serious voices. there's also a great moment of clueless Sherlock that is just adorable. (granted, it's nowhere near as cute as the teddy bear routine)