i386
I would not consider AIX, personally. It is a nice OS for what it is. It does database apps really well, but its network stack is a bit slow and cumbersome. We need both to be fast.
I wrote the Adaptec drivers for AIX, and I found it to be a very different UNIX variant at the I/O level. You could see a lot of main frame thinking in the structure (i.e. more batch oriented than concurrent oriented).
All UNIX's have stength and weaknesses. I don't think it is possible to find one that does it all very well.