I'm not an Angel fan and I don't think that often about him, but I do like Cordelia/Angel a lot. And ok bear in mind this is coming from someone who hasn't seen, and has no plans to watch, the Angel series.
I think on one level, Charisma was pretty close in age to David, so they looked fairly natural together even when BTVS was set in high school. On another, Cordy is just so extravagantly, hilariously mean - and Angel absolutely goes for that. I mean, look at Darla. Dru. Spike. NO it wasn't just the vampirism in those cases - there was a raging dramatic bitch in each of those characters and that's what Angel was drawn to. Or, what he made them into, in the case of his childe Dru, and his pseudo-childe Spike.
And like, pre-vampirism, pre-soulled-penance-guy, Angel was not a nice chill dude. The guy was a mean bastard. Like, high school jock bully mean. He doesn't want someone nice, he wants someone with an edge. Look at how he was immediately drawn to Darla, whose mean girl vibes are powerful. As for Dru, he wants to bring out the mean girl in her. It's about finding/creating a mirror for himself. If Darla is his ideal, his true vampiric love, then Dru and Spike are like self-portraits. With them, Angel shows the world that this is the kind of vampire, the kind of monster/man, he is.
Bringing us to Cordy. Soulled Angel's all penitent now, he longs for goodness to make up for all that prior badness - he longs for Buffy, his new ideal. But wanting Buffy is all wrapped up in denying himself of her. Loving Buffy is a kind of penance and self-flagellation all at once. But Cordy - there's the girl that's perfectly his type. She's not just mean, she's totally unrepentant. Cruel and charming as hell. He sees his human self in her - and even with the soul, occupying this totally new moral position for the first time in his life, he recognizes her and he's attracted to her. Buffy is this constant reminder that he's dead, he's Other. I think Cordelia makes him feel alive.