Derek and Meredith - Grey's Anatomy
While their first kiss we never got to see, their first on-screen kiss was intense. Meredith pinned Derek onto the wall of an elevator in quiet desperation that left all of our mouths hanging open in need of more.
Jim and Pam - The Office
Jim had been pinning over Pam for two seasons even though she was engaged to someone else. But, in the season two finale, Jim’s feelings are finally returned when Jim kisses her, and she returns his embrace in a sweet but passionate first kiss.
Nathan and Haley - One Tree Hill
Nathan might of starting pursing Haley for the wrong reasons, but his feelings for her were as genuine as they come and they are clear as day in their first kiss.
Magnus and Alec - Shadowhunters
Alec was so consumed by his feelings, that he left his bride to be at the altar to passionately kiss Magnus in front of all his friends and family. The look of shock on his parents' faces still gets me every time.
Karolina and Nico - Runaways
The show had been making hints about Karolina’s sexuality and her crush on Nico for several episodes, and it all comes to light on Marvel's Runaways Season 1 Episode 9 when she finally tells (or rather shows) Nico how’s she feels with a beautiful kiss that Nico reciprocates.
Jackson and April - Grey's Anatomy
The two had been friends for years with never ever hint of sexual attraction, but toward the end of Grey's Anatomy Season 8, April confidently kisses Jackson after a bar fight, and the two have sex resulting in the loss of April’s virginity.