Of course I think most people here started with some sort of algebraic/4 function input, but started to suckle on the amazing teet of RPN.
My first RPN calculator was an HP48G my dad had. It was something that I had never seen before. He told me he had no idea how to use it, as it had been many years since he last did. I figured out it's quirks fairly quickly and soon became enamored with it. I used it in my first college maths course, and I have been using HP calculators since.
However, the more modern they get, the more algebraic they get. You went from very basic algebraic support in the 48G, to an entire algebraic mode in the 50G, to an entire menu that's almost always in an algebraic mode, but the HP Prime still has RPN support that I still use.
It's honestly a really nice calculator. It's the smallest calculator of it's power on the market, and beats out anything and everything Ti has ever sold. It's got a hinged tactile keyboard, brushed aluminium, a 16bpp colour touch screen, and a feel of a solid unit with no wasted space, and everything you could possibly want. It feels more expensive than it is, and operates in the same way. It isn't like any other calculator. I always want to have it when I do any maths, as just using it is a euphoric experience.