Which Microsoft OS versions and tuning tools do you recommend to host applications targeted for the following environments?
Note: Dont worry about my hardware. Just tell me what would suit me best considering the environments mentioned above and the need for stability, security and high uptime.
Well I would recommend Any edition of server, 2003 is ROCK SOLID, and I've heard nothing but good things about server 2008. Be mindful that security becomes much more difficult as you move away from XP. (It's not impossible just there are a few changes that usually bite)
Also if you are using WCF in your server, 2k8 has WAS (Windows Activation Services) which can host your service on a non http binding without needing a little wrapper to create the damn thing, you just give it a compiled service definition.