Interesting question.
KQ6 does happen to be my favourite. Yet I do not think it has the best music, graphics, world (I prefer Daventry over the Green Isles), or even story. And frankly, I was much more frustrated by dead ends in KQ6 than in KQ5.
I do, however, think it has the best puzzle design. Which, in an adventure game, is extremely important.
But I think the main reason why it's my favourite is the richness of little details. That was not possible in early KQ games because of technological constraints, and I think I read somewhere about how Roberta would have made more detailed games from the get-go if sufficient memory had be available back then. Don't get me wrong, I love KQ1 being simple the way it is, but...
I played the KQ games for the first time in order, as an adult (never played them as a kid), and after playing KQ12345, even though they are a pretty diverse set of games (and I'm glad they are - who wants to play a clone of an earlier game) I figured that I had a pretty good idea what to expect from KQ6. And to some extent, I did. But I was blown away by all of the little touches. And it wasn't an occasional touch. It was tons of little touches ... and so many things that seemed coincidental at first turned out to be related. For example, I am one of those people who like death in adventure games. In KQ6, you die over and over again figuring out stuff and see the death sequence, but then you finally manage to enter the death sequence without dying. And it's not even obvious that was possible at the beginning of the game! How cool is that? And who knew that a metal nightingale could be so useful ... and I could go on and on. That's the point.
I finished KQ6 feeling like I had a more significant experience than when I first played KQ1. That is in spite of the fact that I first played KQ1 when KQ was totally fresh to me, whereas I first played KQ6 when KQ (in general) was much less fresh to me.
I don't get why some people think that KQ6 is more accessible than other KQs. Okay, I can understand why people may consider parser games less accessible to those unfamiliar with parsers, but what makes KQ6 more accessible than KQ5?