What free email provider do you prefer?

I'd suggest setting up your own email on your own box if you are so enabled. It is your best defence against getting wrapped up in *other* peoples legal issues. That is to say if they want access to email server X they need a warrant and will take it. If your on your own server they're not going to be able to go on a fishing expedition like they otherwise would when targeting other people (or supposedly targeting other people).

The one thing we can learn from Lavabit is that sometimes its best not to rely on others if at all possible. Keeping things decentralized makes it all that much more difficult for our adversaries.

As Chris suggested, the best option for an email solution is to DIY.

If not, I would recommend riseup.net. You can find my recommendation, as well
as a whole bunch of other suggestions, on this thread:
https://trisquel.info/en/forum/what-kind-email-services-are-you-using-and-recommend-and-why

If you want to self-host, try Mail-in-a-Box.

If you don't want to self-host, give Tutanota a try. It offers 1 GB of gratis storage, a responsive Web interface and can send end-to-end encrypted e-mails. Beware, you can't access it using regular e-mail clients since the storage is encrypted. There are free/libre mobile apps available though.