Gene Roddenberry. Or at least he would have on two separate occasions had control not been slapped out of his hands, or if fate hadn't stepped in.
Having read William Shatner's Star Trek Memories and Movie Memories books, it seems the actors thought really highly of Gene's abilities to write Star Trek scripts and fix any script that was sub par during the first two seasons. But at some point after that, he just lost it somehow. Either he forgot what Star Trek was supposed to be, or he forgot what NBC had made Star Trek into during the original series. Case in point: The Motionless Picture, a boring, vapid storyless mess that had Gene's fingerprints mucking about all over it, mostly to sabotage whenever anything didn't go his way. Thank goodness Paramount kicked him upstairs and turned the reins over to Harve Bennett because another Trek movie like that would surely have killed the franchise. The direction that Nicholas Meyer got off the ground in Star Trek II, which was largely continued for the rest of the TOS movies, was what Trek needed to be and, hell, much more akin to the original series than TMP was!
Then, of course, he got his second chance on TNG and that show spent its first year manned with a crew of smug, pontificating, sterile weirdos who couldn't go two minutes without bragging about how much better they were than anybody else. And that show only got good when Gene's health, sadly, began to deteriorate, and he finally passed on. The amazing strides in storytelling that happened on late TNG and DS9 would not have been possible with Roddenberry around. His concepts were just too limiting.
I've just never been able to understand some of the tenets that Gene espoused for Trek: the perfect humanity, the non-military presence, the no conflict between characters. I've seen every episode of TOS, and, I'm sorry, but none of those things existed there. Yes, humanity was better, yes, there was none of the same earth bigotry we had back in the 60s, but there were tons of examples of humans behaving like humans and having to learn. The best example off the top of my head was Lt. Stiles attitude towards Spock in "Balance of Terror." And Starfleet's (or UESPA's, or whatever agency they belonged to that week) agenda was mostly exploration, but they were certainly packing, certainly followed a militaristic hierarchy, and were always presented as a military presence. Gene's refusal to acknowledge any of that shows a complete misunderstanding of his own original work.