Many would say that the second series of BtVS is the most important and well-written. I would be inclined to agree in many ways - the season contains significant story arcs and events which are often mentioned in later seasons. The major events of this season are, obviously, the entry of Spike and Drusilla, Kendra, Ms Calendar; and the whole Angel thing which is, in my opinion, the most important storyline in Buffy history. Minor storylines are also introduced which become of more relevance in later seasons: Willow dabbles with Wicca, Xander starts to move on from Buffy and Giles is shown to feel human emotions! (No offence Giles!)
As well as being important for the serious fan's collection, Season 2 is great for the casual viewer as it provides simple entertainment as well as th plotlines, something which became more lax in the later seasons as the plot took over entirely. Brilliant stand-alone episodes include "Halloween", "The Dark Age" and "Bad Eggs".
The season is beautifully written, especially the final Buffy/Angel face-off which is very poignant and moving. A brilliant season of drama, action, comedy, sci-fi and great all-round entertainment!