Right now, the only 2 realistic platforms for mobile are Apple iOS and Android, and forks of Android like GrapheneOS from there, and not for some superficial reason but because I really don't like it's design, I really don't like Android. I've tried it out before on the Nexus (the flagship before the Pixel), Kyocera, Nokia, and no matter what I try it on... I just don't like it. I do like the design of Windows Phone 8-10, to this day. So it's not just a fanboy thing because I like Apple history but because I don't like the way Android is designed.
Easily the original Macintosh. I truly believe that while graphical user interfaces (GUIs) of the last century would have emerged anyway because that was just the natural path personal computers were headed, they wouldn't have had personality and soul without Macintosh (and Lisa before that). Atari fans can point to it, historians can point to Xerox and PERQ, but they don't have a Happy and Sad Mac, smiling faces, or the friendly personality the Mac offered.
Beyond that, Palm. An Apple Newton diehard could argue the Newton, and maybe someone else could argue for the IBM Simon or early Symbian devices -- but only one had the blueprints of mobile devices that inspired everyone else, including the iPhone and Android. And that was Palm. And I'd be willing to debate that with anyone; the modern home screen layout, low power mode, and even what'd become the 'control center' in other phones were ideas Palm came up with first. After the classic Palm OS, the original Palm webOS pioneered the idea of using cards for app switching, gestures, and multitasking that was ahead of the iPhone at the time.
I also love Be and the Be operating system as a cousin to the Mac. So many good systems (like Irix) seem to be lost to time and my goal as a software artist is to try to remix history and maintain historically inspired projects for upcoming generations (like Gen Alpha) and beyond.
Keypad, which was a simple plain text box in HTML5 I wrote for my first phone, a Nokia running S40. After writing documents out in it, I would paste them into Symbian's notepad and then transfer everything over Bluetooth to my Mac when I got home. Yeah it's super primitive, but you asked which one was my very first one :)Â
However, if you mean my first real project, I'd written a predecessor to lidslate between 2012 - 2015, and lidslate is the recreation of that user interface with a solid Devuan foundation (which is what all my projects are powered by -- except for Condensata).
So as an Apple nerd, one thing that I really wish that I could publish is a visual clone of the classic click-wheel iPodOS (Pixo), but I'm not sure if I'm ever going to do that.
Far as any other historical interface remixes, I haven't worked on them because they need to mean something to me in order to do it, and I gotta love the interface to put time into it. For the black and white interfaces, I think of the connection to the Lisa and Macintosh 128k, and the same with Be and NeXT inspired stuff. Drivestamp and Whistlithium are an interesting break from this, but it's mainly because I've always wanted an XP-like environment on Linux and never really got it.