By most important I think “which one is worst to break on stage” and I think the answer is D. Thick enough to whack the rest out of tune when it breaks, and right in the middle so makes anything you play awkward. There you go, case closed.
Feel like you get the most useful voicings out of A, D and G. The bass can cover the space taken up by the low E. Not much the high E can do that the B can’t.
Both half-surprised and half-expecting the majority to vote Top E as the least important.
I always found most utility with the 5th string for composition though.
When I was first learning to play the guitar (on a steel string dreadnaught acoustic), the g-string snapped, cutting my uncalloused finger tips and my cheek. Needless to say I was disgusted and threw the guitar in a corner for three months. Eventually I picked it back up again, retuned the B and top E and dropped the bottom E to a D, and that guitar has remained a 5 string ever since; I have a small stack of unused G-string packets sitting in a gig-bag, along with the machine head since I figured I could remove that and hang interesting things from the hole.
So yeah. Turns out G’s a cunt and you don’t need it.
Counterpoint - as far as I recall from my nirvana mtv unplugged tab book, you can play the entire solo to ‘The Man Who Sold The World’ entirely on the g string