I run a web development business and have about 10 Linodes now...
I cant say which is the best OS but we have chosen Centos 6, and we use Virtualmin/Webmin to manage it.
I have had this setup for about a year or so, and we have not had any issues really. Nothing I can associate to the OS or Webmin anyway.
Previous/old hosts we have had Cpanel and we DO have issues with that, it seems when we have issues its related to Cpanel. We do have to occasionally restart Cpanel or Apache or something on these boxes, although they are older VPS, with Centos 5.
Eitherway I understand Webmin does not change any OS files, whereas Centos changes OS files and is depply integrated into it. You can remove Webmin and everything works, but you cant remove CPanel. Thats what I read, I have not tried it! I feel safer knowing the OS is unchanged.
You also save on CPanel licenses of course

. Saying that, I love Webmin so much I would pay. I do pay for Cloudmin, thats fantastic at managing mulitiple Webmin servers.
My old server hosts were managed servers with Cpanel, and I often had to raise support requests for them to sort something out. The Linode Webmin VPSs are NOT managed, but I dont think I have ever needed support on sorting out an issue, either because I was forced to find it myself or there was no issue.
The fact is, I feel safer with my unsupported Webmin/Centos Linodes than I do with my supported CPanel/Centos VPSs elsewhere! I certainly have more uptime.
As for Ubuntu... I think Centos updates are more stable and less frequent (again, its what I read so correct me if I am wrong!). So Ubuntu you may have the bleeding edge packages, but maybe they could be less stable than Centos tried and tested less frequent updates. What is important for a web hosting server is stability in my opinion. Centos still release all the necessary patches for security and bugs, and they are frequent, so problem there. Plenty of posts about this online though for more in depth comparisons!