Lumia 520 was my first smartphone and I really liked it's polycarbonate back design. IIRC, it was based on WP 8 though but wasn't upgradeable to 10 (like most devices). I liked stuff like tiles and it's camera though it had its limitations (couldn't playback mkv files for eg without a paid third party app; this was fixed in WP 10).

I also upgraded to Lumia 550 as my Windows Mobile 10 device and it wasn't bad either though the platform was close to its demise by that time. I eventually gave it to my Dad and got a Galaxy J7 in exchange which became my entry point to Android world.