Dave Cockrum helped bring the flagging X-Men comic back to life in the 70's. Relaunching the franchise as The New X-Men, he introduced characters created by him (Storm, Nightcrawler, Mystique, Colossus) that are known the world over today.
I discovered the X-Men during John Byrne's run on the comic but eventually had to go back and read all the Cockrum books leading up to Byrne taking over. This was the Golden Age for the X-Men. The best bar none. You young whippersnappers can stop trying now.
In my opinion, there are 3 guys that are responsible for the popularity of the X-Men today. Those 3 guys are Chris Claremont, John Byrne, and Dave Cockrum.
If you'd like to read more about Cockrum, go here.
If Wolverine were to deliver the eulogy, it would go a little something like this: "Rest in peace, bub."