I credit Gene Roddenberry with being the most significant creative force behind Star Trek. But we have to keep in mind that the guy wasn't very successful outside of Star Trek, and he didn't produce anything successful inbetween TOS and TNG, which was a period of almost 20 years. I've always been under the impression that he was rather "out of touch" with things by the time 1987 rolled around.
Honestly, while G.R. did come up with the concept (although to be honest, it was
Forbidden Planet retooled for TV) - I'd disagree and say
Gene L. Coon was the real creative force that took G.R.'s Star Trek concept and really fleshed it out and expanded it with elements we just take for granted at part of Star trek today.
Rememebr in the original pitch (and early episodes) it was a 'United Earth" ship - Gene L. Coom was the one who expanded this into
The United Federation of Planets (which would explain what Mr. Spock was doing on it, again something that G.R. kind of glossed over - ie WHY would a ship sent from Earth have an alien assigned as crew?)
So, often I think the cotributions of Gene Coon either are forgotten or attributed to Gene Roddenberry.