Dave Cockrum & Bob Wiacek, April 1982.
This was the first issue of X-Men I ever owned: I was nine, so I usually got one trip to the newsstand per week, and a dollar to buy a comic. While my parents were kind enough to support my reading habit, neither of them had much patience with the agonizing process of selecting only one comic from a display of dozens.
I got about five minutes, tops, so generally the one with the most interesting, eye-catching cover won out.
At the time, I barely knew who the X-Men were, and I had never heard of the Starjammers, but there was a lot of really colorful and weird super-people on display in Uncanny X-Men #156, so that was an easy sale.
As far as I’m concerned, Dave is second only to Kirby for character design prowess.