The term "Impressionism" (or, in French "L’impressionnisme") was first used by a critic named Louis Leroy when reviewing a painting exhibition that included a Monet work entitled Impression, soleil levant. The review was harshly satirical and was intended to make fun of the new style of painting represented by Monet and colleagues. We no longer intend any derision when we use the term, "Impressionism" has just come to be a standard term for painting in which the effect of light is often a primary subject. Many other aspects of those painter's styles have also come to be part of the definition of impressionist painting.
I'm not sure who first started using the term to describe the music of Debussy, Ravel and others, but they heard a vague similarity between the way the impression of light is captured by those painters with these composers' focus on tone color and striking harmonic juxtapositions. The term was already not exclusively used derisively, but still had the halo of insult for a lot of people, and I imagine that's part of why Debussy reacted against it so strongly. More to the point, VERY few artists enjoy being put into a one-word box that lumps them in with other artists. The term "neo-Romanticism" comes to mind: a designation that has been explicitly rejected by a lot of the composers it's applied to—including Corigliano, Tower and Del Tredici. It's an uncomfortable thing to have all of your varied works labelled so cleanly and seemingly decisively.
That doesn't mean that these kind of umbrella terms are bad, just that a) they tend to be disliked by a lot of the designated artists, and b) we should always remember that their use is only general and shouldn't stand in for a deeper understanding of each work individually. To answer your question simply: I think the best term to describe these works is "Impressionism," if only because that's the term that most people use to describe them, for better or worse. Anyone with a general knowledge of music of the era has a sense of what the term means, and what it means is "certain characteristic works by Debussy and Ravel." Various aspects of their style can be described and recognized, and any new works that demonstrate a preponderance of those traits can then also be labelled impressionist (or quasi-impressionist, or post-impressionist, or whatever).
