All things considered, probably my biggest influence as a guitarist has been Davey Johnstone - because his parts are just incredible, and because he thinks more like a songwriter/arranger than as a guitarist. His mandolin work is excellent as well. "Mona Lisas and Mad Hatters", "Grey Seal", "The B i t c h is Back", "Funeral For A Friend/Love Lies Bleeding", "Captain Fantastic and the Brown Dirt Cowboy", "Saturday Night's Alright For Fighting", "Bitter Fingers", "Tower of Babel", "Meal Ticket", and on and on and on - listen to those parts, they're absolutely perfect. The guy did the "guitar orchestra" thing before Brian May did, to the best of my knowledge.
When it comes to guitarists that "serve the song", several notables are often mentioned; depending upon who's conducting the list, you might see mention of George Harrison, Mike Campbell, Johnny Marr, Peter Stroud, Buddy Miller. I adore those guys. At the very top of my list is Davey Johnstone; he's long been my personal benchmark.
For that matter, I've always been a fan of the late great Dee Murray. That weird hybrid of Brit blues rock groove-meets-American twang that you hear on
Tumbleweed Connection - well, nobody else played bass exactly like that. Big fan of Nigel Ollson, Taupin, Ray Cooper, Caleb Quaye, Gus Dudgeon,... pretty much everybody that Reg Dwight brought aboard until the records started taking a left turn. Can't say that I've often agreed with Axel Rose, but I certainly concurred when he stated that <paraphrasing, can't recall exact wording>,
"Elton John made the best pop records of the 70's."