I've played Wizardry 8 a couple times but never 6 or 7. I was contemplating doing so but 6's graphics are just so horrible... would I be confused in terms of story and context if I skipped it and started with 7?
I personally really like 6, but the story for 7 doesn't really suffer if you haven't played 6. Its kind of like the beginning of Wizardry 8. I started each game from scratch, without transferring characters, and didn't regret it.
I think 6 is the best in terms of balancing puzzles with the other gameplay aspects (8 does not have enough, 7 is at times way too difficult for its own good in this regard).
Besides that, it can be quite safely skipped - the other 2 games utilize very little of its story elements and the coolest item import is something extremely unlikely to happen without a walkthrough.
Definitely do not skip 7 - that game still hasn't been surpassed in quite a few aspects, as far as dungeon crawlers go.
I loved Wiz 8, but, like Drithius, I couldn't get into the older titles due to how dated they are. Hell, Wiz 7 was released in 1992. That's a 9 year gap between Wiz 7 and Wiz 8.
It would be cool if someone remade Wiz 6&7 in a more modern engine, but I don't foresee that happening.
I went through all three of these games about two years ago, all three are certainly still well worth playing. Hopefully at some point we'll get some of the j-wizardry games on steam or gog.
Wizardry 6 was my favorite by a large margin. I just loved the mystery that slowly unravels and the dungeon design. The puzzles were a nice mix too of being challenging enough that you sometimes weren't sure how to proceed, but not so challenging that you needed a walkthrough (which was good because walkthrus weren't as readily available back then).
I never played 6, but I remember 7 was my favorite game for like 8 years after I played it. Funnily enough, I don't remember a thing about it except the name of some character "Brother T'shober", weird how my brain works.