Depends what you mean by “best”. Personally, I use Lubuntu, as it is an ideal combo of features vs. resource lightness for me. Linux light and Puppy Linux are also among more popular lightweight distros.

Personally, AntiX and IceWM have been the best combo for my old netbook. The lack of systemd does get annoying (some apps directly require it) so I’ve been considering WattOS as a replacement.

gentoo

I use Arch with no desktop environment and it’s fast af.

Best: Lubuntu with LXDE.
Best if you enjoy pain: Arch with Gnome

If you like debian you could try crunchbang++ and Antix.

Or you could install arch or void linux and configure it minimally with xfce or some window manager.

If you want something fresh , checkout nixos. I have had nixos on my desktop for less than a month now and I’m already considering sticking with it for the forseaable future.

I like MABOX Linux. It’s pretty great and runs well on my dumb Chromebook. Puppy Linux is a solid choice as well.

For me it was arch, as you start literally with only the packages you chose. So you can go as lightweight as you want. You don’t even have sudo unless you install it.

There was also some debian image that was barebones and let you just install what you want. But I don’t know if they still offer that option.

Then you don’t even go for a desktop environment, you can just use a window manager, like i3. It’s more lightweight.

The most lightweight I went without going server was with Arch+i3. But having to install everything you need can be tiring, specially because there are many things you don’t know you need until you find yourself without them.

It was a little experimental. For a functional thing you can try debian+xfce (though debian ancient packages can be flustrating), I"m generally fine with Linux mint + xfce or mate. It’s not the most lightweight you can go. But it’s easy to install and lightweight enough that will run in most hardware.

Download BalenaEtcher and burn an .ISO of your selected distro to a USB memory stick - Pop!OS and Linux mint are perennial favorites. Bear in mind that this will erase all data from the stick.

Boot the laptop into BIOS (you will need to check with the manufacturer to see which key you will need to hold to do so) and scroll down to the “boot from device” or similar option. Select “Boot from USB”, save settings and reset your laptop.

If all goes well, and your laptop likes the distro, you should see a bunch of cryptic text scroll by. Don’t worry, this is what Linux shows instead of a loading screen. A menu should pop up, asking you if you want to try out the Live distro, or install the OS. Choose the live distro first, this will create a version of the OS that works from the RAM disk and does not install on the hard drive.

You can now play around with the OS, browse the internet, play games, anything except saving locally to the hard drive (unless you Mount it, but that’s another story). When you are good and ready, you can either choose to dual boot to Linux and Windows, or take the plunge and use Linux as your primary OS!

Hope this explanation wasn’t too rambly. Have fun!