Quite a few people have mentioned that they play The Specials, Madness, Selector etc when asked for Ska music. The answer to this question depends very much on who is asking the question.
Real ska is from Jamaica & some of the Caribbean islands, from the late 60's. Artists such as Prince Buster, Desmond Dekker, Skatalites, Pioneers, Dandy Livingstone etc.
Many of the hits by the Specials and early Madness were cover versions of this old Jamaican Ska. Madness got their name from a Prince Buster song by the same name. One step beyond was another of Prince Busters tracks. The first chart hit from Madness, The Prince, was a tribute to the legendary Prince Buster. The Specials first hit, Gangsters, was originally called Al Capone when released by Prince Buster.
So a long answer to a short question

, if it was someone really into reggae/original ska then I'd play tracks such as Prince Buster-Al Capone, Harry J - Liquidator, Dave & Ansil Collins-Double Barrel, Desmond Dekker-007, Lord Taname-I'm in the mood for ska, Pioneers-Longshot kick de bucket, Skatalites-Guns of Navarone.
If they were wanting pop-ska, i.e. Specials, Madness then the posts above mention the obvious ones, although it was only the first album from The Specials & Madness that had the ska sound and ska covers. The 2nd albums from both groups were very much drifting away from that sound.