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.
The Filipino pride and one of the greatest boxer of all time, Manny Pacquaio!
Manny Pacquaio aside from being a famed and decorated boxer is also a Senator in the Philippines. He was born from a poor family and have even experienced what it feels like to not have eaten any proper meal for days. He had to work and train very hard in order to achieve everything he has now today.
Due to the struggles he went through, he understands completely how some people are treated and discriminated because of social standings. With him winning back and forth boxing titles for the past decade, Pacquaio decided to give back most of his earnings to the community, he have already given out lot of scholarships, medical assistances, donated to many notable foundations and at one point gave someone a house and lot just because he feels what that person has been through.
Pacquaio is one of the very rare few people who didn't forget where they came from. He might only be a sports-celebrity but he is one of the most down to earth celebrity living today.
It is well documented that the most down to earth celebrity is Keanu Reeves. Just google it and then sit down and read all the stories for hours. Here is just a little taste of it
This is a pretty subjective question but I would say either Leonardo Dicaprio because of all the charitable work he does for the environment or Bill Murray because there are countless stories of him just walking through a city and interacting with the locals.