Every King's Quest game except for 7 and 8 has dead ends. In KQ6 they are VERY hard to stumble into, but in KQ5, they are all over the place. KQ6 has the best balance between storyline, challenge, great interactivity, etc. KQ5 is a classic, and my favorite of the series--very high fantasy, but definitely punishing. If you like that sort of thing though, it's your bread and butter. I would say give KQ6 a try if you want to see the best the King's Quest series has to offer.
The remake of KQ1 is pretty much 1:1 with the original (SCI version). The KQ2 remake has a completely rewritten story, and only bears a passing resemblance to the original game in terms of locations and main characters. It's VERY different, but great in its own right.