Depends on how many users you need - if 25 is enough, Server 2012 R2 Essentials (i.e. the new "small business edition") should be OK. Otherwise 2012 R2 standard, or get a 2008 R2 if you still can. Anything server 2003 and older is out of the question (support ending soon / already ended).
If you do not care about hotfixes, technically any old Windows server 2000 would be able to act as a DC, though of course not with the latest version of the active directory.