What do you mean by "most supported"? Almost every vendor offers a $30 to $80 retail boxed version which includes a good sized manual and limted phone support. For more money, you can get more phone support.
In general, though, there are literally TONS and TONS of people, websites, newsgroups, BBSes and LUG (Linux User Groups) who will be glad to help you. I kid you not, 95% of my linux problems I have fixed by surfing the web looking for answers, the other 5% I had to mail somebody or post a question.
Linux.org has links to the 100+ "Howtos", which is a great place to get started.
[shameless plug] I like Slackware. If you're comming from FreeBSD (good for you!), Slackware is what you may be most comfortable with. It's most like 'old' Unix (aka System V). My roomate switched from Slackware to FreeBSD with minimal issues. [/shameless plug]
Mandrake 7.2 is a good distro, but it is very easy to install and run. If you're looking for a challenge, Slackware requires the user to set up most of the system manually.
Why do I even try?
After trying Mandrake and afew other linux distros, I find that Red Hat is the most suppoted version of linux, and has the most ready available driver support and the biggest hardware compatability list. With linux you really need to consider what it is that your doing with your machine. If your doing programing of webpages or graphics, or if you burning CD's, playing games or running a network, give it some thoght as to what you want to do with your box.