Who is the least replaceable member of a band?

[Jonny Come Lately]

Interesting question, and one I can't offer an immediate answer to. So, I'll answer it by looking at five of my favourite bands and determining which members are least replaceable:

Pink Floyd - Neither Roger Waters or David Gilmour were truly replaceable I think - Roger's solo career and David's Floyd (A Momentary Lapse Of Reason and The Division Bell) were probably not as good as what they could have put out working together, with Roger's work lacking musicality and David's work often having weak or average lyrics (though I think Division Bell is an improvement on Momentary Lapse in this respect). Rick Wright is not as irreplaceable although his various keyboards were essential to Floyd's sound and he wouldn't have been easy to replace. Nick Mason I think could easily have been replaced though, the drums aren't all that important to Pink Floyd's sound and he wasn't a singer or a songwriter.

Eagles - Don and Glenn are pretty much irreplaceable as the main singers and the main songwriters in the band. If I'd been around in 1973 I'd have said Bernie was irreplaceable as he was the best musician and was a crucial part of their country rock era sound, but history has shown otherwise - I don't think he was missed at all on Hotel California. Randy Meisner was difficult to replace as a great high harmony singer and a very talented bassist but fortunately Timothy was just the ticket. I love Felder's guitar work but he was ultimately replaceable once Walsh joined, his vocals were not especially important to their sound and he wasn't even that involved in the songwriting processes (despite classics like HC and VOL) - look at the number of songwriting credits Stueart Smith has on LROOE. It's purely hypothetical but would Felder have contributed so much to the album had he remained with the band? I doubt it.

Led Zeppelin - None of the members were easily replaceable IMO. Plant's voice is so distinctive, but so is Page's guitar work, while John Paul Jones was a multi-instrumentalist and John Bonham is my favourite rock drummer so any of them would have been a major loss to the band. Overall I would say Jimmy Page is less replaceable than Robert Plant - I can just about imagine the band with someone else singing, but I simply can't imagine another guitarist playing those songs.

Dire Straits - Clearly Mark Knopfler was irreplaceable - as their only lead singer, full time lead guitar player and only songwriter (save for one early song which his brother David co-wrote, the only other writers credited on their songs are Rodgers & Hammerstein, and Sting), and later producer, they literally would have had nothing without him. The other members were therefore all fairly replaceable although I think Pick Withers on drums was a loss when he left and him and John Illsley were a very solid rhythm section.

Fleetwood Mac - Surprisingly easy for me, I think Lindsey Buckingham is the least replaceable member by far. Not only was he a singer and songwriter as well as being a great lead guitarist he gave them a rock edge (he wrote nearly all of their rockier songs). Stevie Nicks although a great singer and songwriter was not really an instrumentalist - if you removed her songs from the Mac canon I'd miss her vocals but musically it wouldn't make much difference. Christine McVie also has some good songs and provided some good keyboard parts but her some of her songs are a bit too sugary or insubstantial for my liking - most of the songs by her that I like best either feature Buckingham prominently on vocals (e.g. Think About Me) or on guitar (e.g. You Make Loving Fun), whereas I don't care for songs like Tusk's Never Forget. Mick and John, while being an iconic rhythm section, were not singers or songwriters. Frankly, I don't think FM would be close to being one of my favourites without Lindsey. The fact that they needed to two guys to replace him after Tango In The Night (one to sing his parts, the other to play guitar) shows you just how important he was.

Overall then, I'd say that the main songwriter(s) are the least replaceable member of any band as their departure would be likely to lead to changes in the songs the band made (e.g. the emphasis on lyrics compared to the music, the style of writing). However, out of the options originally listed I would go for the lead guitarist - the guitar just makes a slightly bigger difference to me than the vocalist as it defines the band's sound. In short, the lead vocalist might be the first impression I get of a band, but it's usually the music itself that determines whether I keep listening and as I mainly like rock and folk (and folk-rock) the lead guitarist is usually the most important in defining the sound.