Mine was a car (don't recall what), as my Dad built them, so I at first I got the hand-me-downs from him, of which I have a DM600 Mack truck that still sits on my shelf (unbuilt), and I am sure I have parts of a few kicking around here somewhere if I looked hard enough.
After that, I got into aircraft, and then ventured into armor. Only modern aircraft and armor would do, wasn't interested in much before Vietnam. Mildly interested in Allied WWII, but not enough to want to build them, and WWII German or Japanese was completely out of the question....after all, they lost the war.
For the most part now, I am still more or less interested in modern aircraft, armor as well as autos. I do have a few WWII items mixed in, mostly ones used by Canada, and there are even a few WWII German armor and aircraft pieces mixed in. I also dabble in ships, and sci-fi, as well as the odd figure, but rarely as a stand alone.
The one draw back of being interested in many diverse genres of modelling, is I find myself with a collection rapidly encroaching on 600 kits. Of course owning a hobby shop, and being able to buy what you want for yourself at costs doesn't help either!!