I find the Beatles a little overrated. [instinctively ducks] But they are good! They are good! [swats away incoming objects]
It's hard for me to answer your question. Greatest contribution, greatest influence worldwide, greatest person influence....it's hard to define "greatest." The Who, The Clash, The Ramones, Bon Jovi, the Rolling Stones....they could all be called the greatest by their respective fans with little argument.
Since you said personal opinion....Like almost every other music question on the board, for me the Doors are the answer. They were the first to totally integrate poetry and classical music with rock 'n' roll, drop the F-bomb on a record ("The End" from the Doors' self-titled album), break the "3 minute limit" on a regular basis, perfect the stage dive, and be some of the first real "bad boys" of rock and yet have the talent to back it all up. The keyboardist played a whole song backwards on an album just to get a cool effect. The Doors delved into various genres, from sea chantey, to the blues, to Greek epic, to German show tune, to goofy love song, to suicide note. With the diversity, darkness and all the surrounding myths, they stand number one for me.
"Oh tell me where your freedom lies
The streets are fields that never die
Deliver me from reasons why
You'd rather cry; I'd rather fly." -- Jim Morrison, The Crystal Ship
"The future ain't what it used to be." Jim Morrison.