Ok so I’m finally writing my thoughts up on this matter!
Buffy and Angel
I am focusing my attention on Bangel as it is the couple i do know best, so please any Spuffiers out there write one explaining the Spuffy era.
As I am not the best with grammar I have decided to keep it simple by splitting it up into short sections, also so I don’t start my famous rambling and go on and on one point for years
Buffy and Angel first meet in the first ever Buffy The Vampire Slayer Episode (Welcome To The Hellmouth). She does not realise he is a vampire until a mid season in the episode ‘Angel’, where they share a kiss and it forces him to change to his vampire face. After finding out parts of his past they continue to grow close and a passionate relationship builds.
They start dating in early season 2.
Angel: My people -- before I was changed -- they exchanged this as a sign of devotion. It's a claddagh ring. The hands represent friendship, the crown represents loyalty, and the heart...well, you know. Wear it with the heart pointing towards you. It means you belong to somebody.
Then after their first night of passionate relations (Surprise/Innocence), Angel experiences a true moment of happiness which breaks his gypsy curse and reverts him back into his evil side Angelus. Buffy spends the rest of season 2 fighting away her ex lover and her feelings towards him whilst he continues to harass her.
Finally in a dramatic season finale Angelus Spike and Drusilla decide to awaken the demon Acathla and bring about the end of the world and Willow and Buffy discover the curse that Jenny Calendar was working on before Angelus killed her, and so they attempt to curse him again. This fails and so Buffy realises that she must now kill her ex lover to save to world, however just before she is about to kill Angelus and send him straight to hell Willow manages to restore his soul, however it is too late so Buffy faces her hardest challenge to date, having to kill the man she loves in order to save the world!
Buffy: Stay with me.
Angel: Forever. That's the whole point. I'll never leave. Not even if you kill me.
After running away to LA and then returning Buffy still couldn’t tell her friends what really happened that night with Angelus/Angel and keeps it a secret until Giles tricks it out of her to determine the real reason why she left. The just as she was getting over Angel and starting to move on he drops from the sky, and after being tortured for hundreds of years in a hell dimension he acts nothing more than an animal would. Buffy discovers him when looking for a creature who has been clawing the skin off people in Sunnydale. She restores his sanity and decides to keep him a secret which eventually comes out when Xander spots them smooching in the old mansion.
Angel: You still my girl?
Angel: In 243 years, I have loved exactly one person.
After again another year of on again off again, will they wont they romance, with Faith the rouge slayer getting in the way a few times as well, and Angel having to drink Buffy Blood to save he leaves for LA so that Buffy can live a decent normal life.
The next time we see a Buffy Angel reunion is in the series Angel (I Will Remenber You), he had just visited Sunnydale to keep an eye on Buffy yet she was the only one who didn’t know he was there so decided to follow him back to LA and ask him what the hell was he thinking. After a demon attack who restores Angel human side, Buffy and Angel rekindle their love, but when Angel realises that he is not strong enough anymore to help protect Buffy he realises that he must reverse time and never be with Buffy again, to save the world. This resulted in one of the saddest Buffyverse goodbyes.
He pops up again in season 4 of Buffy when he comes back to Sunnydale (The Yoko Factor) to try and mend his relationship with Buffy after their recent confrontation over Faith (Angel, Sanctuary), and ends up exchanging blows with Riley instead.
When Joyce’s funeral he appears again, this time to comfort her as she worries how her mothers death will affect her life. They then share a sweet kiss but both pull away realising they cant have that anymore. He then stays with here until sunrise
Angel: I can stay in town as long as you want me.
Buffy: How's forever? Does forever work for you?
He then appears one last time on Buffy and has a very interesting conversation with a cookie dough analogy (from Buffy) describing why she has Spike in her heart but we Bangel fans still do see that glimpse of hope!
Buffy: Angel. I do. Sometimes... think that far ahead. We both have our lives, but...sometimes...
Angel: Sometimes is something.
Buffy: It'd be a long time coming. Years, if ever.
Angel: I ain't gettin' any older.
The Bangel Wedding