There is never just one with questions like this.
1.) Robert Cray. Of all the players I listen to, he is the one with the most definitive clean Stratocaster sound. His tone is just so honest and pure.
2.) B.B. King. While I admit that I prefer sounds like the one he got on 'Live at the Regal'. I do love his more clean studio sounds. You can really hear everything that is going one with his playing. On top of that. The humbuckers really lend themselves to his playing. Usually I don't really get into clean humbucker sounds but he found a way to make it work.
3.) Mark Knopfler. Like Robert Cray. His stuff is really very honest. Also, it helps that it is yet another example of a great, clean Stratocaster sound.
4.) John Mayer. This one is tough because even when it is clean, you know there is a lot of stuff going into it. If there is ever a guitar tone that sounds "rich" (both in the sense of its texture and the amount of money that goes into it), it would be John Mayer's tone.
5.) Eric Clapton. I am one of those rare types who actually likes every era of Clapton's career but when talking about clean sounds, his work in the early 70's to early 80's is a great example. While not the most guitar solo heavy in this era. Songs like 'Promises', 'Lay down Sally', and 'Wonderful tonight' stand out with their understated and subtle sound that I just love.