[asdfman123]

You could argue that it's one of the most versatile instruments, sure. "Greatest" is completely subjective.

But is it one of the most versatile instruments? You can do signal transforms with any kind of audio input, although it's done more with the electric guitar than any other instruments.

I would say it in practice, it has the most versatile sonic profile.

[solomonb]

A modular synth is more versatile in terms of enumerated signal transformations. Its the ability to be expressive with those signal transformations that makes the guitar+tube amp what it is.

[vegadw]

I'm a guitarist, but also have a modular.

With the right interface, I think the synth can be more expressive. Look at the Haken Continuum or ExpressiveE Osmose - both can be used with something like the Expert Sleepers FH-2 to get MPE data to the modular.

I do see your point, and agree the amount of articulation you can do with guitar is hard to beat, but I do think a synth can win, if the setup is built for it.