Episode 0 of Season 1, which was remade into Episode 1, got me excited about the series. I loved the villain (superb performance), and it had great humor (Dr. Watson character). None of the later Episodes could match, all but the third (and last) of the third Season. The highlight here is the villain, who ranks as the best in the series (Moriarty is rather overdone). He represents great character development and is given a flawless performance.
As a sidenote, of the series regulars, none of the characters was as interesting as that of Sherlock's big brother. It's a superb performance too, the character is such a joy (mature, wise, influential), and he was made all the more interesting to later discover that his intelligence matched that of Sherlock.