A fairly memorable season one episode, I've Got You Under My Skin is quite a revealing standalone episode and a fine example of how a non-arc episode should be. It is the most illuminating episode yet in terms of character with both Wesley and Angel receiving some important bits of character. Angel, at Cordelia's insistence, confides in his grief over Doyle's death and the failure and guilt he feels at having not prevented it.
Wesley on the other hand, we discover, has serious father issues that have shaped much of his early adulthood. The lofty expectations of a father who could never appreciate his son, never for all the effort he put to make his father proud. It is the single most important piece of characterization about Wesley up to this point and will remain pivotal to his character.
The story itself is wonderfully handled and is about demonic possession. To a certain extent, this episode very much homages The Exorcist and is even nicely referenced a number of times. Angel and co decide to help a family whose son has being possessed into a demon by performing an exorcism, but in a magnificent moment, the episode turns the tables to reveal that it was the demon himself (who has previously possessed countless) that was trapped within the body of a boy, one virtually devoid of any humanity. It is a stunningly terrifying moment and by the end of the episode, Angel cannot do what must be done: to save the child's soul.