I had an E36 (328is) and was surprised by how easy it was to work on. The amount of DIYs available online is a huge asset that's not available in every car community. For me, I think the line would be somewhere between and E36 and an E46, and as many have echoed, I'm not as interested in anything after the E46/E39 generation. Don't get me wrong, I appreciate the newer cars, but don't see myself owning one...
We owned a nice low-mileage E36 328i convertible for a while. I loved the car. Did a bunch of stuff in order to get it functionally and cosmetically perfect. There were a few things that challenged my flexibility, but really not bad to work on. Online knowledge base is good. She ultimately decided it was too slow. Guy that bought it got a really nice car.
Even my kid could DIY on his E46, which is a good thing as the car had 250k miles on it and needed ocasional work. My own E38 was also quite easy to DIY most stuff, though the engine swap was a bit larger project that I didn't have time or energy to do. But even that COULD have been DIY'd.