Oh, I thought we were voting for individual scenes where characters had a good laugh (usually at someone else's expense). I had four in mind, waiting for a fifth to be mentioned that would jog my memory. Oh, well--I'll keep lurking.
1. Gary Ewing (KL)'s contagious "belly laugh"--never faked, always a result of him finding something hysterically funny and just 'cutting loose'. Most memorably, he was telling Abby that he'd heard what he'd be inheriting in Jock's will. He was all dour and serious until he said "I guess it was daddy's way of saying HAVE A NICE DAY!!" and he practically doubled over laughing, while Abby just stood there not quite comprehending it.
2. Karen MacKenzie, KL. Granted, she didn't have many opportunities to exhibit her laughter, but it was great when it happened. Abby dropped by Karen's office to needle her about Jill Bennett "sniffing around" Mack. Karen, well aware that Jill was sleeping with Abby's husband Gary by that point, burst out laughing and kept laughing to such an extent that Abby ended her 'visit' and walked out of the office with a puzzled look on her face.
3. Greg Sumner, KL. Like others said, his laughter was usually AT someone rather than WITH someone. It was no small accomplishment for him to use that whole, huge set of teeth to laugh at Abby's latest failed scheme or Laura's latest bon mot tossed at Ruth.
4. Sable Colby, Colbys/Dynasty. Like Greg, she was usually getting the 'last laugh' on someone else, and like they say: he who laughs last, laughs best. The haughtier, the better!
5. Jack Ewing, Dallas. Hearing about his antics on Dallas is a revelation, since I recall DR doing the exact same fake-laugh all the time during his tenure as Grant Harrison on Another World...and it bugged the heck out of me! It had to be a "crutch" of the actor's that he could not shake.