There are several great moments of particularly evil Angelus, or the dark side of Angel. Here are some of the episodes you can find them in:

§ Innocence (BtVS) - Angelus' return after 100 years of soulfulness! His return to his 'family', his snide comments to Buffy, him threatening Willow - complete character turnaround.

§ Passion (BtVS) - He kills Jenny Calendar, leaves her dead body as a gift for Giles, and then spies on Buffy's reaction when she hears the news. He really enjoys it.

§ Enemies (BtVS) - Angelus comes out to play.... but it's only pretence. Anyone else think Angel enjoyed that a little too much? *eg*

§ The Prodigal (Angel) - Newly turned, Angelus slaughters his family (tasted like chicken *eg*).

§ Eternity (Angel) - Angel is slipped a happy drug, and Angelus gets an outing. No real destruction, but snide comments aplenty!

§ To Shanshu in LA (Angel) - Determined to stop the ritual that Wolfram and Hart are performing, and regain the scroll, Angel simply chops off Lindsey's hand. As you do.

§ Are You Now Or Have You Ever Been (Angel) - 1950's Angel isn't much of a people person. So he leaves a hotel-full of them to a paranoia demon.

§ Dear Boy (Angel) - Angelus (and Darla) torment, kill and turn Drusilla in 1860, while Angel gets closer to losing it in the present.

§ Guise Will Be Guise (Angel) - Angel takes the advice of a false Swaami, and gets in touch with his dark side by reeling the man out of the sunlight with a fish-hook in the face.

§ Darla (Angel) - Angelus aplenty, both before and after he gained his soul in 1898.

§ The Shroud of Rahmon (Angel) - The influence of the shroud brings out the worst in Angel, and he chows down on Kate.

§ Reunion (Angel) - Darla's vampiric rebirth and Drusilla's reappearance seem to finally push Angel over the edge, and he locks the Wolfram and Hart lawyers in Holland's cellar with the pair, condemning them to certain death. Oh, and then he goes home and fires his crew.

§ Redefinition (Angel) - Angel continues to explore his darker side, finishing up by setting Darla and Drusilla alight.

§ Blood Money (Angel) - Angel keeps up his vendetta against Wolfram and Hart, menacing Lilah and foiling her and Lindsey's plans.

§ Reprise (Angel) - Angel's nasty to his (ex) crew, gatecrashes Wolfram and Hart's 75-year review (menaces Lilah some more), and finally fights and then sleeps with Darla.