I'm going to go with Peyton Manning as the most down-to-earth celebrity.  He played professional [NFL] football for 18 years and is considered to be one of the best quarterbacks to have ever played the game.  He has been in many commercials as a spokesperson for both Nationwide Insurance and Papa John's Pizza.  And yet for all his accomplishments and successes throughout his life, when I read news articles about Peyton Manning, the one word that I consistently see in these articles is "humility".

Journalists almost always set aside a part of their articles about Peyton to discuss how humble he is, and/or his thoughts on humility.  Many of his TV interviews back up those claims too.  I remember many times throughout his career I would watch on TV as analysts and commentators would praise Peyton's football skills ad nauseum, and I would think that they were over-hyping him just to get ratings and I quickly became sick of it (it also probably didn't help that I was never a fan of the Colts or the Broncos, except during Super Bowl 50).  Then shortly thereafter I would see Peyton being interviewed, and he always looked humble.  I rarely saw him play up his own accomplishments and he often gave credit to other people.  When it came to humility, I genuinely felt that Peyton Manning talked the talk and walked the walk.  Even his opponents often spoke of how he conducted himself with class.  In the end, I realized that I couldn't hate Peyton Manning the person, as he seemed like a genuinely good guy; I had to hate the way that the mainstream media shoved their portrayal of him down everyone's throats.

In addition, Peyton Manning did a lot of charity work during his career and has continued it since retiring from the NFL.  So looking at the whole of his character, I believe that Peyton Manning is the most down-to-earth celebrity that there is.