Faith Lehane: one of the best character arcs of the show, also was used sparely and correctly. Her appearances developed her and the characters relationships around her. Sanctuary is a prime example; character growth for Angel and Wesley in that particular episode.
Ethan Rayne: great baddie, should've been used more
Willy The Snitch: gave the town of Sunnydale some verisimilitude in the early seasons that was sorely lacking in the latter ones.
The Burkles: loving, wholesome parents in a world with mostly bad ones.