Friendships:
Cordy/Angel - I adore this friendship. Charisma and David's chemistry is amazing when the relationship is focused more on their friendship. For some reason they lost all their chemistry when the relationship turned romantic (for me at least, I know I'm in the minority with that statement)
Wes/Gunn - J. August Richards and Alexis had great chemistry, especially in Season 2 when the AtS gang is forced out of the Hotel.
Buffy/Willow/Xander - At least in the early years before everything became fractured within the Scoobies. Alyson and Nick in particular have great chemistry that seemed to work in whatever scenario they were put in.
Spike/Fred - James and Amy worked so well together and the Spike/Fred friendship is one of my favourite Spike-relationships. Their chemistry is cute and light and it brings a real sense of care and trust to the screen.
Romantic Relationships:
Darla/Angel – Julie and David have incredible chemistry, which fills the Dangel relationship with heat, passion, tension and emotion. I love their interactions, in particular because the Dangel relationship has so many levels and the chemistry comes through on every single one.
Buffy/Angel – There’s a reason they’re my OTP. Sarah and David’s chemistry is so strong and so wonderful to watch. It’s why Bangel endures so well; every interaction they have is so chock-full of emotion, heat and love. I always wondered if Sarah and David had something going in real life.
Wes/Lilah – Oh-so-hot. I think Alexis and Stephanie had better chemistry than Alexis had with any of his other love interest (yes, including Fred). This chemistry helped turn what could have simply been a sexual relationship into a multi-layered, rich and engaging ship.
Fred/Gunn – cuter than puppies and kittens and bunnies and fairies, these two were my first AtS ship and it’s all down to the chemistry between J. and Amy. They bring so much love and warmth to the relationship, giving the audience a deep connection and a beautiful ship.
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Damn, how on earth did I forget Victor/Sierra? *facepalm* Dichen and Enver have such amazing chemistry that the relationship was able to come through even when they were in Doll state. They're beautiful, in every way possible and their relationship is full of love, warmth and longing.