Kelthaz wrote:

Or you could work your way up from An Unearthly Child, but I don't recommend it.
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The reason I don't recommend people start with An Unearthly Child and work their way through is simply because so much of Hartnell and Troughton's eras are missing. In fact, the earliest missing story is Hartnell's fourth.
It's easier for someone to go back and watch the existing stories from those eras after they've been intorduced to Doctor Who and have an understanding of the show.