As for which my favourite QfG game is? Hard to say. I like QfG1 (EGA) for nostalgic reasons, as it was the game that first introduced me to Sierra games in the first place (though not to graphics-based adventure games - that honour (?) belongs to the EGA Maniac Mansion).
Compared to QfG1, I like QfG2 for the more involved story, the more developed characters, the more difficult puzzles.
QfG3... not so much, but it serves to build up Rakeesh's and Uhura's respective back-stories, and also introduces some of the most endearing characters in the QfG canon: Kreesha, Salim, Harami, Manu, Rajah, the Laibon, etc...
For sheer inventiveness, atmosphere, story and characters, though, you can't beat QfG4 in the series. Everyone has something to do, a quest to accomplish (with the exception of the poor fighter, who's stuck with "just kill every monster - hurrah!")
I got QfG5 when it came out (Christmas 1999), installed it straight away and played it for about 6 hours straight. (I was much younger and much stupider in those days). :p I liked QfG5 back then for its (comparatively) lush and beautiful background graphics, the setting, the characters (some recurring, some not), and so on. Of course, I also had some pet peeves... the hero's animation was stilted. The thief traps were sometimes much too hard to solve in time. The Dragon sometimes escaped before I could kill it. And don't even get me started about what happened to Katrina.
So to cut a long story short (too late!)... it's not really possible to choose. They're all good, but for different reasons. Sorry.
