To be honest I wouldn't trust any of them for 100%. No one can see how they really encrypt the mails or not and which backdoors they might have. They can write as much information on their page as they want but no one can prove them to be 100% true.
The best way to write secured mails is via selfmade PGP keys. And you store your private key on your own secure computer.
Think about the following: Proton for example offers account mergings after you got a paid plan so you can merge your non paid plan emails to your paid plan emails. During this process they will reroute the old non paid email accounts to your paid account. So in my opinion, there is a risk that they may at least be forced by authorities to reroute Mails to authority accounts.
However I think you really have to do a terrorist act or be El Chapo that they do such measures.
Governments won't do this just to prove your
tax evasion, I am quite sure about this. If you get leaked, then because of the unsatisfied employee of your offshore provider who told the tax guy, they have your face on an ATM camera, your ex wife snitches, you tell someone drunk or on drugs that you are a big offshore bad guy or whatever, but not because of using proton, tutanota or even gmail.
I would still feel safe using proton, not because I trust them for 100%, because I feel safe with them as i know the chance to be leaked on a different way are much much much higher.
But if you want to do highly illegal stuff, than go with your own PGP solution as encryption/decryption takes place on your computer and not somewhere in the clod.