I looked over the list to see what are the earliest Lego sets I can remember owning:
6927 (All-Terrain Vehicle): a communications pod and carrier, in my mind. I think this was the first one I had. I think it may be sitting, assembled, at my parents' house.
6861 (X-1 Patrol Craft): I distinctly remember this one too; it was probably one of the first two I owned.
6890 (Cosmic Cruiser). This one strongly influenced the shape of spaceships I designed myself. I built numerous wedge-shaped craft that were basically scaled up versions of the Cosmic Cruiser. The Cosmic Cruiser separated into two ships (the little one shown in the picture slots into the back of the bigger ship), which I always thought was cool, but I didn't manage to design many of my own with the same property.
6930 (Space Supply Station). This one wasn't as formative. The support stanchions broke way too easily. But, I built numerous supercomputing centers, physics research sites, and power generation centers on the base. The cratered base was sometimes annoying to work around.
6951 (Robot Command Center) may have been a present to Dan, but we both played with it. One of two on the ugly list.
1593 (unnamed?) is the other one on the ugly list that I think I owned. This and the space supply station didn't get rebuilt from instructions all that often. This one was particularly annoying because the rocket nozzles kept falling off (because they were on a 2x2 L-shape hooked onto a 1-wide rail.)
I think I owned one of the rocket launchers, maybe 462 but I really can't recall building the model more than a couple times at most.