I was looking through some old threads that I had posted and I came across this post I made in June 2010 here (on the 2nd page but read through the ranking of Fighting Fantasy books) and I realised that I actually haven't explained what I look for in a gamebook, so here it is.
I guess what your best gamebook is depends on what you want from a gamebook.
Things I like:
Vivid characters and an imaginative setting (Moonrunner, BVP, Chreature of Havoc, Crimson Tide, Spectral Stalkers, Portal of Evil, Slaves of the Abyss Beneath Nightmare Castle and Talisman of Death I think are the best examples of this.)
A well rounded hero, which can be done with a back story or a flavourful set of skills (Moonrunner has a good backstory, Midnight Rogue has a set of skills that fit the character. Master of Chaos has a set of skills, but they are not very flavourful. Taking them away takes nothing away from the story or the character of the hero.)
A main villain who isn't just a 'boss monster' (Karam Gruul is a good example of this. Balthus Dire is a pure boss monster (not Zagor, but only because of the painting.)
Problems that require some thought (How to beat Globus, several situations in Siege of Sardath, Slaves of the Abyss.)
The episodes in the book are part of an overaching quest to solve rather than small disparate episodes(Rebel Planet and Moonrunner.)
A descriptive, well written story (Night of the Necromancer, Legend of the Shadow Warriors, Portal of Evil, Slaves of the Abyss.)
Something innovative Something new that hasn't been done before, either in terms of rules (Starship Traveller) or flavour (Howl of the Werewolf, Night of the Necromancer, Magehunter)
Things I don't like in gamebooks:
A lot of items either because they are all 'power ups' or because the whole book revolves around collecting random junk and then using it later. (Legend of Zagor is the biggest culprit in the powerups category. Basically, if the book has a potion of stamina which restores half your initial level in stamina points, it's a powerups book. Crypt of the Sorcerer comes under the random junk collection category.)
Linearity Just makes the book a big exercise in turning to as many paragraphs as possible. (Trial of the Champions)
Random sudden deaths (I don't mind sudden deaths because you did something stupid, but dying because you picked an option which sounded sensible makes you feel cheated. So does death by die roll. Chasms of Malice is the big offender here.
I'll forgive one from the Crimson Tide because I find it quite clever to turn to 400 towards the end only to be killed off. I'll forgive that one for its cleverness and entertainment.
Flavourless, statistics driven characters makes the book like a computer game. Fight generic orc sk 6 st 5. Later on, fight ogre sk 8 st 10, then dragon sk 12 st 20, then the boss sk 16 st 30. Find a wizard who gives you a potion. This type of gamebook will work better as a computer game (I'm looking at you Legend of Zagor, which I think was turned into a boardgame. Also Knights of Doom, Night Dragon, Curse of the Mummy and Island of the Undead.)
Impossible, unavoidable fights Makes you feel like you can't win without cheating (City of Thieves, Knights of Doom.)
Arbitrary puzzles and random consequences to your choices I don't want to get inside the authors head (Daggers of Darkness, Chasms of Malice.)
The book is just a bunch of encounters There is no thread tying the plot together (Scorpion Swamp)
Tedious adventure sheet management/map making It may be important to the game but when it is the game, it becomes boring (The Sorcery! series for all its strengths suffers from this. Scorpion Swamp is an example of a book where too much of the game is making a map.)
I think that's it. People's opinions on books will differ on how important they rank these for and against points. If someone likes fights and collecting potions etc. then Legend of Zagor will probably be a favourite.
Lots of books with good points usually have a few flaws (It may get tedious deciphering the code in creature of Havoc or you may not like the setting in Moonrunner) and lots of bad books have redeeming features (Scorpion Swamp has a spell system.)
I think Battleblade Warrior is an interesting gamebook because it has very few good or bad points. It is a perfect example of a perfect middle of the road gamebook. I think it leans slightly to the good side with some background and some interesting characters and scenarios.