All in all, i prefer the early series. The reason is simple:
The early format for Knightmare was like a Televised Computer Boardgame, almost like HeroQuest (if anyone remembers that) and the airbrushed scenes looked very much like cardboard rooms. I liked the way the dungeoneer would walk through a door, and as far as we knew, the next room could be anything. The later series - 5 in particular - are WAY too linear.
I first started watching KM when S4 started, but i remember catching a bit of S3. Even aged 6ish, I thought that the older KM i had only briefly seen was way better and far more scary.
Later on, many of the "hated" features of the new format began to work well, S6 in particular as it was tension filled and really gripping.
I dislike S5 as the outside locations are too "normal" (i thought the previous purpleness was cool as it reassured us that this was a different reality/time/place. Also S5 really over used the eyeshield, in S4 and the later series it seemed to be used sparingly, when a player leaves a room. In S5 entire scenes are through the shield