Technically speaking, HTC Glacier and boy was it a total POS like most early android/HTC h/w to this day, but I faffed about network unlocks and other fun things from Moto RAZR V3 and beginnings of XDA.
Business Nokias were particularly fun to "root" and such.
Xiaomi Redmi Note 4, and the first ROM I installed was LineageOS, and this smartphone introduced me to the world of custom ROMs. Since then when I buy a smartphone, I first see if this smartphone can be installed with a custom rom or not.
A Samsung Galaxy Y when i was 5 years old.
Yep, i'm not lying, i did install the rom but it didn't boot, so it was took to service and it was working again
I used my old Galaxy Tab Lite 3 7.0" to install the rom and my old Galaxy J1 at that time good ol' times