Ok so I've decided to go into depth in each member of Queen's musical style and significance in the band as some members (namely the first one that I'm going to start with) often don't get credited as being important to Queen.
John Deacon
Unlike the other three members of Queen, John Deacon was most influenced by soul and funk music as opposed to rock music, particularly American rock and roll. It was something of a rarity for the bass guitar be used as anything outside of a background instrument that just added thickness to the music. John's playing however broke that mold by using his bass as a rhythm guitar on many occasions and it even being featured as a lead instrument, particularly from "The Game" onward.
During their time in the 70's John's bass playing was among the most intricate and tricky in music. Utilizing walking bass runs, sharp note changes, and his trademark runs (just to name a few things) his playing was much more difficult than heard in his most famous riffs such as "Another Bites The Dust" and "Under Pressure".
Generally John would play with the right index and middle fingers but would use a pick and sometimes his thumb on certain songs. He even played a double bass/upright bass on the song "'39", where he taught himself to play the instrument in an afternoon.
Some of his most prominent and finest parts are on the tracks:
"I Want To Break Free"
"Dragon Attack"
"A Kind of Magic"
"Breakthru"
"Under Pressure"
"Another Bites The Dust"
"The Millionaire Waltz"
"Liar"
"'39"
"You're My Best Friend"
"I'm Going Slightly Mad"
Aside from his bass playing alone John Deacon also played guitar and to a smaller extend keyboard as well as wrote several of Queen's biggest hits. Also he was the only member of Queen to never sing lead on a song and wasn't ever credited on vocals for any Queen song. The reason for this was the John felt that Brian, Roger, and Freddie were all such strong singers that he simply couldn't keep up.
His compositions were often the more poppy songs in Queen's catalog, which stemmed from John not having the rock roots of the other members. Some of the notable songs he wrote for Queen are:
"Another One Bits The Dust" (Queen's highest selling single in the US)
"You're My Best Friend"
"I Want To Break Free"
"Spread Your Wings"
"Back Chat"
"In Only Seven Days"
"You And I"
"Need You Loving Tonight"
He also wrote the riff to "Under Pressure", which is of course the backbone of that song.
John Deacon is the least famous member of Queen for several reasons, that he never sang lead and that he never embarked on a solo career being two of them. Queen was a unit and as overlooked as John often was/is as a contributing member of the band he was a vital part to Queen and very musically able in his own right.