#6 SQ4 Pros: Narrator voice, graphic. Cons totally wasted time travel potential, weak story, dead ends, frustrating puzzles
#5 SQ3: Pros: puzzles. Cons: no story at all, astro chicken!
#4 SQ6: Pros: good story and narration, voices, no dead ends. Cons: aimless wandering at begining, copy protection puzzles.
#3 SQ2: Pros best story and puzzles in EGA SQ era. Cons: difficulty, dead ends.
#2 SQ1: Pros: being the first, story, fairy easy compared to other "first part" Sierra titles, humor. Cons: dead ends, mini-games.
#1 SQ5: Pros: Best plot and characters in series, you are a captain of space craft!, only one(?) dead end. Cons: no voice acting, maze.
Actually SQ5 has more than one dead end:
-Enter the EVA pod the second time (when you're about to board the Goliath) without the laser thing you need to cut your way into the ship.
-Enter the EVA pod the second time without having retrieved the liquid nitrogen tanks from Genetix.
-Enter the EVA pod the first time without the spare oxygen tank.
-Pressing the self-destruct button before you need to. You can abort the self-destruct, but everyone else on the ship will have already left the ship.
Also, I don't know if this counts as a death or a dead end or both, but if you spend too much time wandering around in the EVA Pod the first time, you'll have either not enough oxygen or not enough fuel to return to the ship with Cliffy.
Actually SQ5 has more dead ends than SQ4, so it's more than odd you list dead ends as a weakness in SQ4. The only dead end in that game is if you don't write down the time code for SQ12 and you don't keep a saved game back there.