If you have a budget with actual money in it, I'd say Redhat. It's the OS the devs use and develop on. Suse would be a very close second and really is only preference.
The flip side of it is a $0 budget, I run Ubuntu 6.06 (8.04 upgrade this weekend) and have been for nearly a year now. If you are going to purchase the Network Edition, Ubuntu is a supported OS and works well. Ubuntu also has the benefit of a huge user base and massive community support or Canonical can provide commercial support as well.
If you want Redhat, but have the budget of Ubuntu, then CentOS is the way to go. It is currently not supported, but from what I read the Redhat rpm's install fine (CentOS is 99% ish compatible). Also it looks like the 6.0+ version will be supporting CentOS if the beta's are any indication.
Mail | Dell 2950III | 2x Quad Core 5420 | 8gb RAM | 6x 146gb SAS RAID 0+1 | Red Hat 5.3 | Zimbra 6.0.10 Network Edition
Test | VMware ESXi Whitebox | Phenom II Black 3.2ghz | 12gb RAM | 6x 1tb SATA RAID 0+1 | CentOS 5.4 | FOSS, Not in use now