Definitely Riley. Never liked Riley. I have to say though that picture of Spike is creepy.

Of these 4 characters and their pairings, Spuffy was the only rival pairing which still existed at the end of both series. But honestly, I don't think it was any character who was the biggest threat to the Bangel relationship. It was the loophole in his curse which meant he could lose his soul, followed by his own insecurities and belief that Buffy deserved a "normal" relationship.

I don't think any of them were, to be honest. It was obvious in S5 and S7 that Buffy and Angel are deeply connected; they never broke up because either of them wanted to and their feelings for each other never faded.

ALL THE WAY SPIKE!!!!
I read in the Buffy comics that she is dating Spike and she told Angel. He gave them space but say it wont last because of who Spike is, which I believe because I'm a definite BangelFanForLife

To be honest, I don't really count the comics as canon. Some of the stuff I've heard about them sounds more like the writers pushing their limits now they can more than sticking to their true characters.