Which is the most overall 'compatible' encryption type?
A lot of (non-US federal) clients I work with request a shared Dropbox or Box for file transfer. If they want to feel "extra secure", non-tech-savvy clients in the past have enjoyed getting hardware-encrypted flash drives the most because they like how "simple and easy" it is to handle them. I don't necessarily endorse these approaches; as mentioned in the comments, marketing influences what your clients want, regardless of their requirements, but I have rarely been in a position to change their minds.



More grizzled war stories follow.

Sadly, the situation is more like "Client #1 wants MP3s, Client #2 needs WAVs, Client #3 will only talk to me if we go listen to concert music together", etc. I don't think you will find any particular format will scale to an arbitrary number of clients, who all have (or believe they have) specific needs. 

Here is a sample of the scenarios I have encountered in the past decade or so:


Scientific data (C code, Fortran-77 code, text, Excel spreadsheets) sent to my lab:
password-protected DVD, client would send password in e-mail 
Financial data from an insurance company: set up secure FTP server with developers on their side
Transmitting data within a company itself: Data is put into a MySQL database and I was asked to "please delete the data in a few days"
Financial data from a bank: Sent us an email that opened a web application that required a password so that we could download some data
Getting data from a university partner: Mailed us a USB-C drive with a keypad to enter a password, which...was written on a sticky-note


I'm not sure what kinds of clients you have, but I guarantee you that you will have one that will surprise you with what they want soon, if you haven't already had this happen to you!