IMO, the best version of Office is Office 2010. According to me, it is the version that looks the best and has the most tools for being productive. Office 2007 closely follows 2K10. It also looks beautiful but has less number of useful features that I use frequently.
Among earlier versions, Office 2000 and 2003 are the best as they have a simple toolbar-style interface and are easy to use with a number of useful tools, still, they lose to Office 2007 and 2010. Later versions from 2013 onwards are not on my list as they are ugly, slow (the slowest being 2016) and bloated. 2013 is somewhat tolerable, but nowhere near to even Office 2000.
EDIT on 16 June 2021: I tried replacing 2010 SP2 with 2007 and I honestly feel that 2007 offers the same speed, for a smaller disk footprint but while cutting out on some features, which are useful but not the ones I cannot live without. The real best version is Office 2010 TP (Build 4006).