Depictions of Diana has varied a lot. If we look at HG Peter's original Diana, she is tall and slim, with some muscle tone but not bulging in any way. I think it's clear that he was inspired by the flapper ideal of the 1920s. If we are to look at modern artists, I think Nicola Scott comes quite close to that.

With that in mind, I think I'll have to point out two key characteristics for a good Diana: she has to have a clearly feminine body (note that this goes way beyond putting titties on their chest, and influences nearly every part of the body), and she should be proportionate. Some muscle tone is good, but it should more be hinting at strength that can be expressed when it's needed, rather than hulking.

And if we look at Gal Gadot, I'd say she comes very close to the way Peter drew Diana: tall, slim, with a hint of muscle tone. Slim hips, that go with a slim body shape, and shoulders that hint at strength without making the rest of her look small.