The Metal Gear Solid games. There are so many laughs that can be had with all of the subtle humour, but one of my favourites will always be distracting the guards with porno books.
Hitman: Blood Money
In the 'A New Life' mission, you have the option of disguising yourself as a party clown. Agent 47. As a balloon making, red wig wearing, squeaky shoe-pie-in-face clown. I couldn't keep a straight face as I strangled the target.
Assassins Creed
It's perfectly ok for the Trouble Makers to shove you into things, but the instant you touch them everyone throws a shit-fit like they're an endangered species or something. But you know you smiled when you stabbed the bastards with your hidden-blade, sync bar be damned!