Which is the most overall 'compatible' encryption type? What is the "MP3" of encryption formats today, which hopefully will make it easy and compatible to send to other recipients (not via e-mail), without them having to download and learn a software suite just to unlock what I sent them?

I work commercially and a part of it means sending people the end result files (usually multimedia files on the larger side). I do not work for a steady number of clients, so I need to deliver to all kinds of people where neither side knows in advance about how we prefer things done. 
Most people I encounter in work aren't tech savvy when it comes to secure file transfers, and many express loud irritation from having to 'unlock' transferred files through some process they've never even encountered before. Their emotions about this tends to associate with me, as a content-delivery service, which might hurt business in the long run. But customers absolutely do want the files delivered 'securely'. That irritation, and wish for safety, are contradiction of course, but you wouldn't want to be the one to tell them that.

Some have also found their holy graal, in using AES256 or PGP or some other fairly known method, and here I come with something that doesn't correspond to that. Or, in some cases we both may be accustomed to 3DES encryption but we use different software to encrypt/decrypt, meaning my 3DES encryption won't unlock through the other guys' software even though it is the same basic encryption formula. Or the software needed to decrypt isn't available on the other person's platform.
People might also be in contact with other people who delivers files to them, meaning in the end they need to have multiple software suites installed and learned just to deal with all types of different encryptions and solutions. Sometimes topped off with a short temper. 

Therefore I'm looking for advice/opinions about the probably most 'overall compatible' way of locking medium to large size files for sending over the internet, which might reduce the need for someone to adapt to me, or me to him/her. MP3 files can be used by virtually everybody. What is the MP3 of encryption, today?

Thank you for any thoughts and/or tips
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!