Considering what she's been through and her status, Britney is the most down to earth. I don't think she even realizes who she is and whenever someone mentions her achievements to her she just gets embarrassed.
There are a lot of down-to-earth celebrities though. Pink, Kelly, Adele just to name a few.
I wouldn't say Gaga is down to earth, because she herself said she somewhere in between the earth and fantasy

But I think she has a very nice girl, even though I had my period of thinking otherwise (BTW era where I felt like the fame kind of got to her).