Angel wanted, and sincerely tried to be, a good parent to infant Connor, and if things hadn't gotten all screwed up, I think that he could have really gotten the job done properly. He failed completely with adolescent Connor, of course, but that wasn't his fault. It was just a question of circumstances far beyond his control. It did seem that he was on the way to getting a second chance toward the end of Season 5, and things looked as though they might actually be working out in IDW's "After The Fall" arc, but then the rest of that mess, followed by Twilight, and now with Angel in London and Connor (supposedly) in LA, what progress that he has made just may be fading away for lack of attention.
But as to who actually did the best job, though, even she was never actually perfect at it, I think that my vote would have to go to Joyce. She wasn't only Buffy's and Dawn's mother, she was the unofficial "mama" for Willow and Xander, and, strange as it sounds, for Spike, who shed many a tear in many a cup of hot chocolate at her kitchen table. And she even made sure that she had the little marshmallows on hand.