Richard Belzer has set various records of variety and longevity for playing Det. John Munch.
REGULAR
Homicide: Life on the Street (1993-1999, plus a TV movie in 2000) Law & Order: Special Victims Unit (1999-2014, 2016)
GUEST
Law & Order (1996, 1997, 1999, 2000) The X-Files (1997) The Beat (2000) Law & Order: Trial by Jury (2005) Arrested Development (2006) The Wire (2008) 30 Rock (2011, 2013) Unbreakable Kimmy Schmidt (2016)
Those are just the times he played that particular character; he's played law enforcement in lots of other projects. Kind of surprised The Simpsons never got around to including him.
Today I learned that he and Henry Winkler were cousins.