Our favorite used to be Windows 2000 Pro, because it didn't spend a lot of time getting in our way of booting up and running automated applications.
Then, Microsoft pulled Windows 2000 last year. So we moved to XP Pro..after some pain in getting rid of most of the "were Microsoft, and we are going to think for you" eye candy and automated autoconfig BS, we again had a stable OS to build on, or so we thought.
But having been burned, we started one of our new digital signage projects last year based on Slackware Linux...and we are quite happy with it. Yes it took longer, but we don't have to worry about MS pulling the rug out from under us. We don't have to worry about losing our development investment with Linux.
Apple's Steve jobs pulled a similar stupid stunt when he pulled the plug on the Power PC and all the development around it. We had built products around that too, but after having our products rendered useless by Apple's decision, (not once but twice, remember Nubus?) we'll never ever develop for Apple ever again.
What MS doesn't get about companies like mine is that there is no way we'd ever build a dedicated box or appliance application on Vista. The premise is a joke. If MS had any sense left, they'd keep XP around so that the OEM market had something to work with that wasn't just a collection of glorified myopic and incompatible eye candy.
Money may buy friendship but money cannot buy love.