"Spin the Bottle" is a bit like Angel's version of Buffy's "Tabula Rasa", on the surface. But underneath, it is teeming with so much more.
In an ill conceived attempt to retrieve Cordelia's memories, the gang use a spell that backfires horribly. Everyone, except Lorne, reverts back to their teenage selves.
This gives us great insight into the backgrounds of characters about whom we don't know much before they came to the show, such as Fred, Wesley and Gunn. Cordelia is a breathe of fresh air in her old Queen C persona. Angel is once again an innocent man, Liam, who is out of place in LA, far from his home in 18th century Ireland and ignorant of his vampirism. Gunn and Wesley's interaction is perhaps the best of the episode. Their characters differ so much, their bickering is hilarious.
Of course, this episode is not filler. Wesley deduces that their situation is like the Cruciamentum from Buffy's "Helpless", and that they must find the vampire. Alas, when Lorne wakes up and reveals that Angel is the vampire, Liam acts like he thinks he should and the rest turn on him. Connor shows up to save the day.
Lorne brings everyone back to normal, but Cordelia's memories do come back, to disastrous consequences.
The format of this episode is unique. Lorne breaks the fourth wall multiple times, even referring to the commercial break at one point. He narrates the episode to a nightclub audience. There are also a couple very cool scenes in which he narrates directly in the middle of the action, such as when he sums up the major points in the Fred/Wes/Gunn happenings.
One of the best episodes in a season that, while it may not have the best overall arc, had many very good standalones.