Hear me out
>in terms of what can have the most negative effect on your game
group > adventure/scenario > system
a system can be bad, but it's easy to sidestep the issues, dismiss retarded rules and come up with bite-sized on-the-spot homebrews
a badly written or thought out adventure can be a bummer and is much harder to fix, especially on the fly
however nothing can ruin a game like "that guy" of a GM or player
>in terms of having a positive effect on your game
adventure/scenario > system > group
I literally never seen a player that could single-handedly make a game much more enjoyable
a fun and well designed system is fun to engage with what more can I say
but most importantly a well designed adventure is what makes the game fun
PS
for open-world games adventure/scenario and system are combined into system