I think it really depends on what you want to use it for. Also, I think that people will have different biases and experiences.
I think personally that if you are just doing server stuff only, debian or ubuntu both have good package support and also very good package and also install from source HOWTOs. If you want cutting edge package support (with some breakage expected) I would recommend Fedora.
Best overall supported, but without latest Linux kernel, is of course from source (or a binary release) using the XEN 2.6.18 kernel (or the one that is currently directly supported by the Xen Source software group) from
xen.org.
Things will get much better once all domU support has been merged into the mainline Linux kernel. Has anyone heard about a serious push on paravirt_ops and the like from any of the major players (RedHat, Novell, IBM, Citrix, etc.) that would get full domU Xen support? I know Jeremy called for it at the last Xen summit, I wonder if companies are taking action?
I would like to hear what others recommend too.
Best Regards,
Todd