My favorite story arc is having two seperate worlds collide and merge. The story follows two characters that have all the pieces of the puzzle, but they don't really know each other. Its even better when one charactor is astonished by the other character. These are the most dramatic moments I have encounterred. Extra tasty when one character arrogantly underestimates the other for the whole story beforehand.

Breaking Bad, where Hank finally discovers that Walter is Heisenberg. Arsenic and Old Lace, the psycho murderer dude discovers that his sweet old aunts are not so innocent and helpless. The Expanse, Detective Miller and Holden finally meet 2/3 of the way through the book.