What is the lightest distro for using Debian/Ubuntu softwares

Which distro uses lowest system resource (when installed on hard_disk/usb.) where I can install software from Debian OR Ubuntu repository?

I found Quelitu which is based on Lubuntu, but uses less memory as given on their site. There are lighter distros but most of them are based on Slackware. I don't have much experience so want some expert opinion. Thank you in advance.

-Ripu