First two seasons of Twin Peaks are his masterpiece IMO and his most watchable.
Those are some of the best characters of any film/tv show ever.
From there I would go to Lost Highway next for a stronger dose of the more out there stuff.
For me, the second step would either be The Elephant Man or Mulholland Dr. -- many of his works tackle very dark subject matter and include sexualized violence that can be downright disturbing to watch, but those two omit those elements. The Straight Story is much lighter, but largely lacks the surrealism Lynch is known for.
it's not like i'm not used to watching long movies and i would call myself some form of cinephile, but for some reason Twin Peaks felt unbelievably slow.