Watching TOS reruns and the movies with my dad on Sunday afternoons when I was a child. Soon afterwards followed by TNG with my older brother. Then VOY was released when I was about 9 years old and that got me hooked for good. For some odd reason, I completely missed out on DS9 until much later in life - I started watching it in my early 20s. Once I watched that, I did a full rewatch of every show of the franchise, including TAS and I still don’t know what to think of it. :D Long story short, Star Trek made me a much better person and had a huge impact on me very early in life.
I’d say VOY, ENT, TNG, DS9 & SNW, but keep in mind I’m a bit anti mainstream in terms of Trek recommendations.