Angel/Buffy
I felt like Willow genuinely got over Oz. She moved on. She realized she's a lesbian. She thinks of Tara as her great love. She would not go back to Oz if they were both single and nothing was in the way. That relationship was meant to be temporary.
But Bangel. My goddess.
Angel started dating Buffy when he was 270 yet he could honestly say she was the only person he ever loved. He was alone, existing but empty of everything but guilt and hunger, no longer even thinking of himself as a person. And then there she was. She inspired him to be a hero and gave him perfect happiness. He once told her "Loneliness is the scariest thing in the world." His love for her got him through one hundred years in Hell. His first word top side was her name and he may have remembered it longer than his own. He literally dreamt of marrying her, wanted her as his wife, to start a family with her, to be the father of her children. Darla was the paramour of Angelus but never truly his girlfriend. Buffy was. At nearly three centuries he found his first love and she turned out to be his soulmate.
He was going to commit suicide just to stop himself from ever hurting her again. When he broke up with her, he thought he was giving up the only happiness he would ever have. He didn't know what would happen (friends, fatherhood, a business) on AtS. Leaving Buffy was the hardest decision he ever made. It was like he was Olaf from Frozen and actually got to touch sunshine and now he's back in the cold and he knows he's a snowman and would melt away but it was worth it because her light and warmth felt so good and right and like home.
And when he left it wasn't to another country. He couldn't be that far from her. He didn't want life experiences or to find himself. He moved to her hometown, the first place he ever saw her, just two hours in case she ever needed him, he'd always be there, in the shadows, the way it was before she pulled him into her world and made him feel human.