I haven't got all of the demons that have ever featured in Angel in here for much the same reason I don't have all the demons from Buffy in here.
Mohra Demon
Fought against Angel, it's blood has regenerative properties.
Wolfram and Hart
Wolfram and Hart are a law firm, now all laaw firms are generalised as evil but not to the extent of Wolfram and Hart. Most of Wolfram and Hart's clientele is demonic and they have a serious problem with Angel who seems to kill some of their best clients.
Darla
Sire of Angel. Member of the Order of Aurelius. Darla was staked by Angel in order to save Buffy. In the 1st season finale of 'Angel' she is resurrected in human form. Drusilla turns her. She gives birth to the only child ever to be conceived by a vampire, let alone two. She stakes herself so her child can live.
Drusilla
Sired by Angel, she sired Spike. Formerly girlfriend and lover of Spike she left him for a chaos demon and again later for a fungus demon. She has both visions and the power of hypnosis. She's slightly insane and has been known to have hallucinations. She was brought in by Wolfram and Hart to turn Darla.
Holtz
Daniel Holtz was a vampire hunter after Angelus and Darla. Angelus and Darla took out their sadism on his family but killing his wife and baby and turning his daughter. A demon approached Holtz and informed him he wouldn't get the chance to ger revenge in this life but he would help him. He placed Holtz in the 21st Century. Holtz stole Angel's son and took him to a demon dimension. he thought up the ultimate revenge, to poison Angel's own son against him.