I have finished all of 6-8 through one party. It did feel an actual journey through all these games, and seeing how time and technology evolved through them. I was going to rank all three games, but that would be unfair as they all had their all interesting quirks and details to them. However, the peak for me was 7. That game is so unique with the pseudo-open world at the time, and all the faction relations. It offset the emptiness and desolation of the cities, and made the world feel alive, but dangerous to traverse. It had some of the hardest challenges Bradley ever threw at me, but I resolved them by going somewhere else and continuing to progress, and I never really grinded at all until the last area before the Savant fight. It has the most brutal difficulty curves, but I enjoyed it for challenge. Bradley turned me into a masochist.
8 was also very good, and I liked the flexibility it had with difficulty, ensuring that if you messed up a build or something you can still progress no matter what. Some of the dungeons were not up to par with those of 6/7 like Mt. Gigas and the mines, but it felt like they were focusing on an overall experience rather than smaller, confined dungeons. I felt disappointed a bit with the factions, as there was less variety and less interesting encounters you could do in 7. It mostly felt like they were quest givers instead of the illusion of living, breathing creatures. The combat was a little below that of 6/7, due to the jankiness of the movement (I kept bouncing of enemies when I went at them at an angle, and Trynton NPCs would block the passages annoyingly enough). Movement was still cool because it added another layer of strategy with positioning and not getting surrounded by many enemies. I did feel it could be expanded more than just funneling in enemy by enemy through a chokepoint.
6 is the most primitive, but I definitely started to enjoy it more after I left the mines. The Temple of Ramm was absolutely brutal, and I did find myself grinding a bit, but that is the only part besides the beginning I did that. I was about to give up, but I eventually got to the end. Retrospectively, I do see it as a solid basis for which 7 expanded on it.
I am glad I went through all of these. I am going to give 1-5 and maybe some of the JP spinoffs a try sometime, and see how they compare.