I hope its not too late
As a software engineer who has worked for more than 20 years developing multi-million line code applications to operate across all of HP-UX, AIX, Solaris (Linux, DG-UX, Dynix, and some other particularly archaic OS-es - anyone out there done significant development on NCR MP-RAS? - well the list goes on and I'm showing my age), I reckon I can offer a reasonably balanced opinion that Solaris has always offered by far the best environment for hosting applications of the bunch.
The amount of grief that segmented memory architectures provide and the lack of proper multi-threaded application support at the os-call level are hopefully consigned to my nightmares now, but HP-UX and AIX always trailed Solaris in these respects. Solaris just makes it so much easier to develop applications that can fully exploit the available system resources. Okay, perhaps the Sparc processor line hasn't always been top of the performance pile but at least you know you can maximise your performance from it.
Sun caused me considerable professional pain though when they withdrew their Solaris x86 support years ago - this really set the clock back - at the time it seemed like such a brain-numbingly stupid move (and of course it was!) - I just hope it isn't too late to see Solaris, now such a well threaded environment with transactional filesystem and fully integrated profiling, take its rightful place as the operating system of choice for enterprise application development.
P.S. In case you are wondering, no I don't work for Sun or have any relationship with them.
P.P.S. Please Jonathan get Solaris out on to the desktop so I don't have to develop on Windows any more (build more device drivers!)...
Biting the hand that feeds IT
