I am looking for the same thing too. My experience: discard Xapo, their fees increase proportionally to the BTC value.
And as they said above, it depends on you are BTC investor or just a faucets user. I use faucets, earn around 40-50 $ per month. And if you are going to use faucets, Xapo really sucks.
With the adjustment of xapo when it comes to incoming and outgoing transactions those amount that will be on their list will be eaten by them. And when it comes to the wallet with the lowest transaction fee, I don't think that there's one that is existing as most of the wallet providers are allowing us to adjust the fees.