Much of that depends on what you have already as well as what you need. For instance, Windows 7 would be a great candidate. However, if the PCs you are wanting to install it on are a few years old you may end up having troubles and then having to pay much more than anticipated in order to get those machines up to the needed level (if you even can).
Also, your needs are important. If each user needs to be able to use a certain legacy piece of software, then you’ll need to either stay on an older version of Windows, or shell out extra for a version of Windows 7 which includes XP (plus, a PC that can handle hardware-assisted virtualization). So, we’ll need to know what things they are going to need to do.
If it’s just basic email, document creation, etc, you could consider some distributions of Linux, such as openSUSE (or SLED as a paid, commercially supported option). Using Linux applications such as LikewiseOpen will allow those stations to authenticate against your existing Active Directory. Naturally, you need to take your department’s level of Linux expertise into account (as well as plenty of testing) before you embark down this particular path.