I love the first few like Deathtrap,talisman and firetop mountain, but purely for nostalgic reasons. i personally think the later ones are a lot better.
House of hell, was fantastic and probably the most difficult book to solve, because it consisted of some great puzzles you had to solve.
Appointment with fear Where you make a super hero and choose your powers...i shouldn't have to say anymore, great book.
Creature of Havoc The hardest to figure out, with great puzzles, you even have to learn weird new language, i think this was probably the best, when i look back.
Spectral Stalkers you are either going to love this or hate it. It involves time traveling around looking for a lost artifact, while your enemy's stalk you through the different worlds.
Ive always wanted to attempt to write one of these type books, and think they are a great way to encourage young people to gain an interest in reading.