What, no SCO? I kid, I kid. But seriously...where's *BSD? And you should probably change RedHat Linux to just Linux, in all its flavors, as pruad kinda pointed out (especially since when it comes to Linux I'm a slackware guy). That said, out of your list (and based on my own experience), I voted for Solaris. stealth edit: looks like mine's the first vote.
Buy?! I won't mention that you're including non-Unix operating systems in your list of Unix operating systems, that'd be pedantic...
All kidding aside, the three "big" systems you list are closely wedded to specific hardware. What kind of setup are you planning? A single server? A cluster? Lots of external storage? How will the entire system be monitored and administered? Who's going to admin the box/es? (each hw/sw combination comes with a completely different set of monitoring and admin tools, very vendor-specific. Finding someone who knows a particular system is a large part of the equation). You obviously don't have to answer those questions here, but those are the kinds of things you need to be thinking about. Also, even though I recommended Solaris, be aware that for some inexplicable reason Sun makes it very difficult to buy their hardware. Unless you order their basic boxes off the web, you have to go through a VAR, and many Sun VARs are notoriously unreliable. I know several medium-sized companies (webhosts, financial service firms, etc.) which wanted to use Sun equipment, but switched to other hardware because they couldn't get the VARs to close the deals. And we're talking hundreds of thousands of Dollars in each case here! So...lots of things to think about.
Not enough choices, also you didnt mention what you are using it for. General hosting? Centos is my choice.
Guys we are planning to buy that for our office purpose and we have only these options...anyways thanks for your valuable suggestions..