This is a tough one, probably the toughest of the contest so far for me. 176-178 is probably the best the entire series has to offer in terms in terms of sustained gripping action; it's tight, intense, and moves quite quickly, unlike certain other *cough 345/Clash cough* BO episodes; as usual there are a few logical inconsistencies, but they aren't particularly egregious.
Moonlight Sonata, though is in a class of its own. It's just so... ridiculously good... (not a very imaginative phrasing, I know). The case is very good and does a great job of setting the tone for the first 60 episodes or so. It was after watching this episode (technically, episodes, as I watched the dub) that I became truly hooked on the series. The power of nostalgia compels me to vote for it.
Detective Movies Contest 2013: Information Thread Current Faceoff
"The energies of our system will decay; the glory of the sun will be dimmed, and the earth, tideless and inert, will no longer tolerate the race which has for a moment disturbed its solitude. Man will go down into the pit, and all his thoughts will perish. The uneasy consciousness, which in this obscure corner has for a brief space broken the contented silence of the universe, will be at rest. Matter will know itself no longer. 'Imperishable monuments' and 'immortal deeds,' death itself, and love stronger than death, will be as though they had never been."