I'll start by quoting Buffy herself, stating that she loved Angel more than she'd ever loved anyone in her life.
I'm also going to explain my preference by criticising her other pairings. Angel gets criticised for making some decisions on her behalf, but I think the others are worse.
Firstly, I've a problem with her being paired with any normal human. Angel broke up with her so that she could have a normal human life - but she isn't a normal human. She's a slayer. Realistically, she's not going to survive long enough to grow old with someone. Same with being with someone who can give her children. The only known Slayer who had a child left her son effectively orphaned when he was just 4, and that's if she survives her pregnancy with all the attacks and apocalypses she experiences.
Secondly, Riley was just no good for her. In my interpretation, she had some trust/commitment issues after Angel, which left her unable to risk falling as deeply in love again, in the short term at least. She was holding herself back. Riley noticed this but interpreted it wrongly. This led to his jealousy, inferiority complex and...