While on Linux you sometimes have to pick between apples and pears (Oranges being just a bit too different). Picking an Illumos distribution is more like picking dark or brown chocolate.
I’m currently using OpenIndiana and I’m overall very happy with it. However I have some issues with OI, being that KVM is more or less broken for me. (I did not experience this in OmniOS when testing.)
For now this is not enough for me to switch, I have faith it will be fixed in the near future. I tried and failed myself, but I should probably give it another go at some point.
Anyway lets cut to the chase. Overall they all provide more or less the same experience, excluding SmartOS because it is geared specifically to be a hypervisor.
If you are looking for a hypervisor, SmartOS wins hands down.
However if you are looking for a generic server distribution I would recommend OpenIndiana.
Overall OpenIndiana has an active community and that is very friendly and inviting. The documentation is somewhat lacking in places. But asking on IRC will usually get your problem fixed or at least get you pointed to some documentation elsewhere.
OmniOS is very similar to OpenIndiana but lost due to the poor documentation and lack of a community. However it is probably more attractive for organizations because there is commercial support available.
DilOS also requires mentioning, it looks very promising. It is a serious candidate if I ever decide to switch. But for now it’s newness is holding me at bay.
Check out this wiki page for a bit more background information where I based this article on.