Archlinux
Why you choose it?
It and its derivatives are very popular (better and broader software support), the wiki is rich and detailed, the AUR is useful for managing niche software.
Running for how long?
Got a new PC last year, so technically only a year. My work computer had an arch install running for 5 years without issues until I was forced to switch.
Which DE?
KDE. Popular, just works for most stuff, did 0 customization.
Dual booting it with windows or pure linux?
Dual-booting W11. Completely disabled updates. W11 partitions are on the right side so that I can easily absorb the space into my linux install if I decide to nuke it at some point. The only things it has installed on it is Firefox, Discord, and League of Legends.