There is no best Linux or there wouldn't be 500 Linux distros. You have to try a few and see what you like. Some choices are between distros that install with everything (Ubuntu), distros where you compile almost everything from source (Gentoo), distros where you compile everything from source (Linux From Scratch), distros with package management and application repositories (Ubuntu, Debian), distros with a base and then compile your own with no package repo (Slackware). It's kind of hard to describe in one paragraph. I guess you are only talking about stuff that will run on Intel since that's what nasm is for. But there are other distros and other hardware platforms.
You have a nice setup with plenty of ram so the easiest thing is to install VirtualBox and then grab a few Linux distros and try them out. Check out distrowatch.com for a lot of helpful info and links to many distros. Then there is UNIX and the freely available OS that run on Intel include Solaris, OpenBSD, FreeBSD, and NetBSD. I'm pretty sure nasm will work on all those too.
I use Slackware and have for about 10 years. There's no better Linux in my opinion. Plenty of people agree with me. But plenty of people also disagree with me