Finn and Rachel - Glee
Finn and Rachel were funny characters in general. Glee was always a comedy, but Finn's goofiness matched with Rachel's over the top personality made them one of the best duos on the series.
Fitz and Simmons - Agents of S.H.I.E.L.D.
If Fitz is smiling, it's probably because he's around Jemma. From best friends to husband and wife, this relationship provided an opportunity for the audience to smile, even if it did come with its fair share of angst.
Chuck and Sarah - Chuck
There's something extraordinarily goofy about Chuck that always gets goofier when he's around Sarah. While it would seem that she would tone down his personality, she does the complete opposite.
Josh and Liza - Younger
Josh brought out a whole different side to Liza that she didn't even know still existed. Whether they were doing secret handshakes or skateboarding on the streets of New York City, they spent almost all their time together laughing.
Oliver and Felicity - Arrow
Felicity's quirkiness mixed with Oliver's mysterious persona changed the entire tone of Arrow. Instead of being another action-packed DC series, Olicity's interactions made it into something more.
Jonah and Amy - Superstore
Jonah and Amy are funny on their own, but put them together and they'll have you literally rolling on the floor laughing. They both have outgoing personalities that can't be contained.