My riding school used Honda CB125's for CBT and Yamaha MT07's for DAS. But between my CBT and DAS I had my own Yamaha YBR125 for about 8 months. I loved that bike and would go out most evenings, taking food and coffee with me. I would ride our and find somewhere nice to sit on a warm evening and have my coffee. It was a great learning experience. So I would say that is the bike I properly learned on.
Back in the 1990's I almost got my licence. I happened to go to a car show at the NEC and opposite the hall was a motorbike show. There was an offer for cheap motorcycle training during January, presumably because it was so cold and wet that no-one else wanted the course. So I took the course more as an experience than wanting my licence at the time. Unfortunately I failed the test and never had any motivation to take it again until much later. But I think we had some kind of Honda CB back then. The training was very different to today, and we could ride the bikes home at night. I think it was larger than a 125 as well. Maybe a 200 or 250? But I distinctly remember riding that home in the dark, and the magical feeling of being out in the open, with lights everywhere, and lighting up the road whenever I put the indicator on. Although I didn't take my test again until decades later I think that experience planted a seed. Strangely I don't remember being bothered by the cold and wet despite it being January and I would not have had any decent riding gear. I guess it just bothered me less when I was much younger.