I think Angel was Buffy's first epic love, not her first love, that would be Pike from the movies (and comics).
Angel was Buffy's first everything, I think in a way killing Angel broke something in Buffy that never really healed, apart from all the atrocities Angelus inflicted on Buffy.
But also she just smiles a lot more when Angel’s around, despite every heartache they went through with each other. They’re the type of couple I feel who can spend years, decades away from each other, to steal Oz’s line to Willow, Buffy would turn the corner and there standing in a crowded street would be Angel and Buffy wouldn’t be surprised at all.
Riley was Buffy's normal boyfriend, the one where she tried to have a relationship that had a modicum of normality.
After all the grand and epic feelings with Angel, being with Riley was easy and maybe something of relief.
Maybe Buffy thought she had a chance with Riley until everything came crashing down on Buffy with Joyce's cancer, and then Riley turned into the kind of boyfriend who needed validation just when Buffy was barely holding it together. And then he came back married, showing off his awesome marriage just when Buffy was depressed.
This one I have a lot of mixed feelings about because they had smoking chemistry but Buffy started a physical relationship with Spike at the lowest point of her life, and at that point, Spike was the only one who could make her feel. But, uh, I can't really look beyond the attempted rape. Maybe if there were more time and distance between that moment and when they met again. My feelings for the relationship between Buffy and Spike is conflated with how much I didn't enjoy season 6 and how much it was part of Buffy's depressive spiral. SMG and James Marsden did have a lot of chemistry, though.
Buffy has a lot of complicated relationships with their own flavor of screwed up and at one point or the other, they all tried to kill each other.
So, I think the best solution is: polyamory or Buffy surprises (or not, Willow has a betting pool) and she ended up with Faith instead.