I voted for 6 but the first one I ever played was 4, and I'll always have a special nostalgic fondness for it.

I started playing through the whole series myself in the spring. I'm still working my way through KQ VI -- played it on release but never finished it, it seems -- and have been taking a break before going back to work on the difficult ending path. I think it solved a lot of the problems of KQ V, with much better voice acting and a more interesting environment, and much less of the "here's a rectangular map with some fairy tale characters sprinkled around it" approach of the early games. But I also feel like the series began to get a bit overblown as time went on -- the simple style of the first few games has its own charm.

If anyone's interested, I've been writing about these on my blog in some detail as I play each one:

KQ I (1990 remake)
KQ II
KQ III
KQ IV
KQ V

For me It's a 3 :D, I think It was a really original game back then.

That constant "tension" of being caught doing something that You wasn't suppose to be doing... that "sneak outs" of the house while the wizard was out, always having in mind his inminent return.... how you work hard little by little to gain your freedon... I liked that King Quest a lot.

I played it in Amiga, like most of the sierra games I've played, and for me, It was an unforgettable game

Regards