Aside from BigTrak, it would have to be the TI-59. But the Ti-59 was rather difficult for kid my age at the time and I relegated to using it as a bike computer. I'd cruise the streets and pretend I was getting orders from base and spying on people and recording data. Like a tricorder of a sort.
Then came Basic Programming on the VCS. I did things with that that scared everyone.
My skills improved slightly when I got an Apple II. Later I became infatuated with Lunar Landing simulations when I got my TRS-80 Pocket Computer - 1. Then I went back to the Apple II and did BBS & MicroModem II programming.