If you were confortable with Red Hat, and don't need much bling, I'd go for RHEL or a clone like CentOS (simple, robust, long life; but very conservative in the software versions shipped). Near (but today probably almost unrecognizable as Red Hat of yore's descendant) is Fedora (latest stuff always, simple to manage; life of a version is around a year).
If you don't mind moving to another distribution, any of the "big ones" (Ubuntu, Debian, OpenSUSE, ...) should work fine, the software they are built of is the same (mostly). But check the points that are important to you in management (how long does a version stay supported, any maintenance costs, do you have people confortable with it at hand for troubleshooting, ...)
Of non-Linux stuff like Solaris I don't know, migrated off Solaris some 12 years back after some horrible "incidents" due to software with glaring, well-known security vulnerabilities that had stayed without fixing for more than a year. And my experience with Solaris back then was that to make the system halfways usable for development you had to install most of GNU and some other assorted pieces. Dunno if that has changed any.
If you work much with Oracle stuff, perhaps best support would be on their own Solaris or on RHEL (or a clone like CentOS). But check their recommendations.
