What usb-class for a device intended to be as future proof as possible?
You probably want a CDC serial port.  I'm going to disagree with laptop2d here, although HID drivers (e.g. ones for a usb mouse and keyboard) are common, I don't see how you would use any of the standard drivers for serial-like communication.  You could of course write your own HID driver, or maybe there is a company out there selling them.

Windows versions from Vista onward (and probably earlier too) include a usb CDC serial driver called usbser.sys.  There are as far as I can tell two major versions, one in windows Vista/7/8/8.1 and one in windows 10.

For the first version, you need to provide an inf file, to link your USB VID/PID to the driver, then windows will load it.  The driver is a bit rubbish, it makes no attempt to recover from transient disruptions on the bus, and we've had trouble getting through European standard immunity tests with it. Due to a driver bug, it is not possible to recover a dropped connection purely in software (!!).

The windows 10 version is better.  If you set the right device class and subclass (02/02, IIRC), windows will load the driver without an inf.  It will still have a yellow ! in device manager though, so for a professional look you'll still want that inf.  If you use an inf, it will have to be signed, and a code signing cert will set you back somewhere under £100.

You could also buy comercial driver from e.g. Thesycon. They are much better than the built in windows one, and they will provide customised distributables. Costs ££££s though.

I understand linux support for CDC serial ports is good, no driver needed (though maybe some udev rules), and I have no idea about Mac.

This all assumes you do the USB yourself on a USB equipped microcontroller, using the appropriate libraries, and then sort out a driver.  Another option would be to work with someone like FTDI.  They can probably provide drivers if you use one of their usb/serial chips.