Use AGPL. Else they will take your code add a little bit to it, do not share it and sell it to you.
No need to add the License in every File, something like this in the README is from a legal site enough:
LICENSE
This work is licensed only under the GNU Affero General Public License version 3 (AGPLv3). See the LICENSE file for details.
Video: Who wants you to think nobody uses the AGPL and why
If u don’t want to do it does not matter which permissive License u use: CC0, MIT, BSD they are basically all the same.