I don't have a favorite. I do not feel like anyone that I am aware of has made proper investment to make a quality development app for mobile due to the low market demand. While development is better than on a calculator I think they are below my expectations.

> What's your favorite free programming environment for commonly used smartphones?

Termux

  pkg install python
  python
  print('hello')
  ctrl+D

Haven't tried these, but have seen them recommended:

Acode

Termux + neovim

Termux + code-server (vscode-like, accessed through phone browser at localhost)

I like Codea for iOS, though the free version has a soft-limit at 500 lines. If a project gets bigger than 500 lines you can still run code but it'll nag you to upgrade.