Ubuntu and/or Mint. I usually trade off and install the other one than what I have every few months, but I think my current Ubuntu distro is going on 2 years. Although I have had other HDD’s with Mint I’d swap out for every once in a while. I like both, but I like Ubuntu more and probably use it twice as much as mint.
Arch is cool for playing around with Linux and really getting your hands dirty. I think that’s what I used when I tried building my own custom distro (I failed that time, but I will try again).