I also thought that 5 was the best Space Quest. I found 1EGA and 2 to be mostly dull myself. (1VGA had a lot of charm, though).
3 was great, but it didn't really go anywhere until halfway through the game, and sort of just flopped around aimlessly until then. 6 had more or less the same problem.
4 was great in the beginning and endgame, but the middle bit felt a bit aimless.
5 is probably the only SQ game that felt to me like it had a cohesive plot and a consistent progression of said plot, as well as the requisite weird characters and spoofy jokes. About the only thing I could lament about it is the lack of Gary Owens.
And speaking of cheating...
Spoiler:You actually *have* to cheat on the test to progress in the game. If you put in all the correct answers without cheating you still fail the test. I wonder what that says about StarCon.
Peace & Luv, Liz