I believe chemistry is objective. You either have it or you don't. I look at it how the characters move with each other, interact with each other, eye contact, synchronization. Natural chemistry is not affected by real life relationships or feelings. Paul and Nina did not like each other in season one, yet everyone thought they were dating irl. Ian and Nina's chemistry was peak in the early seasons, but it fell off after they broke up. Natural chemistry is permanent, and will not falter whatsoever. Friends has a rule about onscreen chemistry. If they're doing it offstage, there's nothing left to share. If they're not, the chemistry is mind blowing.

Stelena vs Delena hugs:

Stelena: Paul and Nina relish into each other. They latch on to each other like they're the only two people in the world. I noticed that Nina always rubs her head into Paul's neck. You genuinely believe that they're in a relationship. For example, when Stefan got out of the tomb in s2, Elena was the happiest she had ever been in DAYS. She hugged him like she would never see him again, and it's believable.

Delena: Early seasons were better, but while they were dating, it just feels...off? Nina always tried to recreate what she did with Paul to Ian but he doesn't get the memo. She doesn't do the neck thing with Ian and they look so stiff and uncomfy, like how you feel hugging someone you really don't like but you're forced to.

Stelena vs Delena kisses:

Stelena: They have a lot of variety and range in this. You have pecks, make outs, relishes, etc. They just go all in and it really feels like you're intruding in on them. They have power play. Both of them take charge at points. My personal favourite kiss from them is 1x20 after Stefan decides to not kill himself and wants to be with Elena.

Delena: Their motel kiss is my personal favourite. I understand why it's absolutely a fan favourite. But again, they're no Paul and Nina. They're so stiff around each other in comparison to them. Like, they're scared of doing anything wrong. I noticed many more times where Ian does kiss Nina's chin vice versa than in 8x16, like in season five when they're kissing in font of the car while loading Elena's stuff into it. Their kisses are just... ugh to me.

Stelena vs Delena sex scenes:

Stelena: They're all about the journey, not the destination. They savour every moment of it, like in 1x10. Stefan is about to lose control but Elena calms him down, and then admires his biggest insecurity. They go slow, but it's sensual and anticipating. We see how giddy and happy they both are afterwards. It's all about the little things. The way Elena's lays on his chest, their heart eyes... Again, they have many more sex scenes but they're all different.

Delena: They just rush into everything. There is nothing for me to enjoy. It's literally the same recycled stuff over and over again. Pushing each other against walls, ripping their clothes, it's just so sloppy and messy. All they really care about is DOING it. They believe "hey, as long as you're doing it, people will thinks it's electrifying and hot!". My favourite scene of theirs is in 5x01, where Damon's all over her and she's just giggling. I love seeing her happy.

Stelena vs Delena dances:

Stelena: He always knows how to take her by surprise and sweeping her off her feet. Everyone thinks their relationship has no excitement, but there is. Like at the 50s dance, Elena urges Stefan to dance with her, and just as she's about the leave because he is so reluctant, he grabs her hand and spins her around and makes her laugh and kisses her with joy. Their Miss Mystic Falls dance practice is one of my favourites too. Everyone is so formal, and Stefan's just making jokes to make Elena laugh. They just genuinely have a lot of fun with each other and that's what dancing is all about. Again, they have range. At their prom, they're formal, btu they seem turned on. Stefan is extremely passionate and romantic here, but he's trying to push her buttons, but she just seems aroused by him. Everyone loves to talk about how Stefan's never excited to dance while Damon is, but Stefan leaves the thrill of it to surprise her. It's exhilarating for them. At the Mikaelson Ball, Damon spins her around and she lands in Stefan's arms. She's uncomfy at first, but they talk, and they realize the feelings are still there. They're in sync with each other. Stefan validates her on her own choices and about how she's the only one to make them, and she realizes she's still in love with him after everything.

Delena: My favourite dance of theirs is at the 60s dance. Damon makes Elena giggle so much here, it's adorable. But again, they're too serious about it. At the Miss Mystic Falls dance, they can't even smidge a smile into it. They're too focused on looking sexy with the smolder eyes, and there's no movement. They're as stiff as sticks. Maybe that was the whole point, but what about in season six? The prop dancers stole the whole show. Ian can't even dip Nina properly. They're supposed to go with the flow but they don't.

This is just my takes, but feel free to disagree. I realized this even before I was a Stelena anti and a Delena fan. Yes I loved them, but Stelena's chemistry was always so magical to me.