I consider vocals an instrument, and would say it would be #1. I know some bands try to find a sing-a-like, but its so rare that the new guy is better.
I would say the next one is the drums. Rhythm is so important, and very hard to duplicate. Even a lead guitarist can mimic things note for note.
I would say bass is the easiest to replace.
Jazz bands would agree about the drums but disagree about voice.
At least you are keeping the same instruments. What about replacement like substituting a cornet by a trumpet or a clarinet by a saxophone? That will definitely alter the sound.
I’ve been listening to some late 50s Philadelphia-style soul-jazz organ combos lately, usually sax/Hammond B-3/electric guitar, drums optional, no bass. The organist can do that with his feet.
But the classic joke is “the piano player can do that with his left hand.”
Also, bluegrass and especially “newgrass” bands have no trouble with propulsive rhythms, and they don’t use drummers either.
And we can further replace both piano and organ by an electronic synthesizer…
What kind of band(s) are we talking about?
Marching bands don’t usually have any place for vocals.
Some jazz bands are built around pianos, some around trumpets.
Bassists are indispensable for a rock band (The Beatles) except when they aren’t (The Doors.)
You mention The Beatles. I would say vocals were the most important. Same with The Doors.
A jazz band with no singer won’t have that problem. I would say the drums by default.
As would surf-rock bands.
John Mayall did just fine without drums in his “Turning Point” LP.
I don’t think any instrument is absolutely essential in bands, since you can always find examples where they weren’t used.
The Dixie Dregs never had a vocalist (and Soft Machine did without one once Robert Wyatt left), while the Persuasions did away with all instruments altogether.
In a general sense I would agree #1 voice (I actually hate music without vocals, it’s missing so much emotion, seems so empty) #2 drums. Cowbell would be hard to replace however it’s only really needed in the first place because it’s cool to say more cowbell. Though some music stiles would require a certain instrument, I think many are replaceable.
Substitutions for common instruments can lead to wonderful combinations.
The original lineup of progressive bluegrass band Crooked Still consisted of vocalist, banjo, string bass, and cello…rather than typical fiddle. Later on the cellist quit and was replaced by a violin, and they lost that unique sound.
For me, it all starts with the drums and bass. Gimme a good rhythm section and you could put any kind of shit over it and it will sound good.
Seems this is diverging into different discussions, but from what I can work out, the OP is about swapping players of the same instrument, rather than changing the actual instruments in the band?
In which case, I’d say it’s massively dependent on how distinctive a given musician sounds. Without Carter Beauford, the Dave Matthews Band is going to be a radically different proposition…but, well, it wouldn’t have been hard to replace Tony McCarroll in Oasis and have the band sound the same. I mean, OK, they did replace him, and sounded different, but that’s only because they wanted to. It would not have been difficult to copy his sound.
I’ve been playing the drums for 30 years and, to an extent, I instinctively agree with you because I know what I can hear. But ultimately I reckon it’s down to the musician, case-by-case, not the instrument.
If you haven’t already listened to them, the guitarist Grant Green’s early sixties recordings with the organist Larry Young are amazing.
I agree. If the musician has a very distinctive sound, they can be hard to replace to the point that the whole band changes. The Dave Brubeck Quartet doesn’t/didn’t work without the 4 key players. Take away Joe or Gene and the rhythm falls apart. No Paul, no soul. And Dave is/was sui generis.
And the drums, and the bass, and the guitars, and the singer…etc etc…depends on what kind of music you plan on performing/making. Everything is replaceable.
Ever see Luca Stricagnoli with playing three necks simultaneously? Amazing… U2 - With Or Without You - Luca Stricagnoli - Fingerstyle Guitar - YouTube
awwww jeeze…ALL players are necessary …
I vote Banjo. Can be rhythm and melodies at the same time.
I feel like you can interchange a euphonium and a trombone (I play both - with the same mouthpiece!) but if you want the sound AND the slide, you gotta go with the trombone!