The first formation of King Crimson included Michael and Peter Giles with guitarist Robert Fripp. Later keyboardist Ian McDonald was added and he brought with him a lyricist, Peter Sinfield, who would help to define the signature of King Crimson's sound. Peter Giles was later replaced by Greg Lake who became the band's bassist and lead vocalist.
The original founding musicians were heavily influenced by jazz and classical sources. There was an open scorn toward formulaic pop music which Sinfield and Fripp often expressed back in those days. This underscored their fondness for complexity and unconventional elements. There was a lot of improvisation in their early work which reinforced the jazz influence.
Main Creative Force
In the earliest days, much of the creative tone of the band was supplied by lyricist Peter Sinfield who, oddly, was seldom actually a musician with the band. He almost never performed on any instrument or sang - just wrote words. But looking at their career as a whole, the one constant element throughout is guitarist Robert Fripp. Without a doubt, Fripp is the core identity of King Crimson.
Most Likely To Be A Mainstream Rocker
From the earliest days of the band, there was an open disdain for the commercial. They made it clear that complexity was a hallmark they sought to achieve all the time. However, as you look at the career of Greg Lake, it is obvious that he is very comfortable flirting with the mainstream. His ballads with ELP were the most likely radio hits and his solo album in the waning days of that power trio consisted of mostly commercial sounding stuff. So my vote is for Lake. Certainly Giles, Sinfield, and Fripp have never flirted with commercial appeal.
First Hint of Prog Brilliance
Let's be honest here: King Crimson was never anything but prog. They came out of the gate writing complex instrumentals and fantasy-themed lyrics. In most ways, their first album was both thier first hint of total prog-ness as well as their prog pinnacle, so with that in mind ...
'Court of the Crimson King' was released in October, 1969. It had elements that connected it to the psychedelic rock which was at its peak at that time, but it was first and foremost a progressive album. The title track, with Lake's vocals, was something of a prog landmark for years to come. It certainly presaged the sound of Emerson, Lake and Palmer. Many musicians in England, particularly, were enamored with the album. Besides the title track, the song '21st Century Schizoid Man' has had an enduring quality and some have seen it as prescient of the grunge era that lay two decades in the future. King Crimson has produced 12 more albums since 'Court' but that remains their most identifiable work and a classic of progressive rock.
Influenced By Them
Most progressive bands that emerged in the 70s were influenced by King Crimson. Rush has cited them specifically. Fripp is noted as a guitar theoretician and has even taught guitar workshops. Consequently, the reach of King Crimson through other bands and musicians far exceeds their peak popularity worldwide.
The band has had 19 full-time members over a 40+ year period but Fripp is the one constant. KC gave Greg Lake to Emerson, Lake and Palmer in 1970 which extended many elements of their sound, through Lake's lyrics and vocals, to a much wider audience. Members Ian McDonald and Boz Burrell went on to found Foreigner and Bad Company, respectively. The band also included Yes drummer Bil Bruford as well as John Wetton who would sing with the bands UK and Asia. Fripp founded a school called Guitarcraft and released a quirky album of mass acoustic guitar known as 'Robert Fripp and the League of Crafty Guitarists'. The complexity and oddness that summarizes King Crimson has continued on both with the band itself and various influences. They never enjoyed any popular success beyond their first album and despite their influence on prog, have virtually zero chance of landing in the RRHOF.
My Personal Take
If progressive rock has ideals, then King Crimson is probably their embodiment. Their devotion to complexity over simplicity, obscurity over popularity, and rebellion over commercialism is their enduring signature. The fact that their first album has two such amazing songs that endure to the present is a testament to their skill as composers more than anything else. I do not think King Crimson really deserves a hall of fame type slot, but I also am totally convinced that Prog Rock as it exists today owes an enormous debt to the influence of King Crimson and it is Fripp and Sinfield who deserve most of the credit for that.