I've used/owned various G&P (RIS, barrel mount) and the VFC. The barrel mounted versions are the most accurate (the RIS one sits too low and ends up looking funny). The G&P ones come for either TM front end or GP front end: the difference is in the type of delta ring you're using (more specifically the barrel nut itself);
TM having their own delta ring going on for their barrel setup. I don't know how the newest M4 is, it may or may not have change. The S-System also has some weird collar, not sure if/how an M203 fits.
The "G&P front ends" use a realistic barrel nut (tube with spurs, see pic below) and so those M203s mount like the real deal. King Arms, Systema (PTW), and probably others all use this type of barrel nut/delta ring. As mentioned, CA requires its own mount, and AFAIK ICS uses the TM style delta ring.
Proper barrel nut
You can buy them as aftermarket parts, but if you have some other proprietary handguard that might faggle up your setup.
Also in the vein of realism, if you want QD capabilities for a barrel mounted M203, there exists a QD front mount which allows you to install and remove the launcher without tools.
M203 QD adapter