Sometimes it's the guys who can crack a joke at the right time who make the biggest impressions on us.
For me, there are 4 great comic relief characters.
Sokka, from Avatar: The Last Airbender
Kuwabara, from Yu Yu Hakusho (I tried to find a video, but could not)
Bender, from Futurama (not that he needed to lighten the tension much, but still)
And Russell Crowe's portrayal of Javert, from Les Miserables (2012)