Overall this is an interesting question, which could be answered one of two ways.
Firstly, we would have to ask if the Brotherhood as truly 'free' to vote, without the local branch providing 'recommendations' to faithful Christians. And if it was like that, I would imagine that the local branch would instruct Witnesses to vote for the political party, the political philosophy or the political candidate that offered the best set of kick-backs for the Organization. Which actually could change from country to country.
However If Witnesses were free to vote according to their own consciences, then I feel that most would vote for left or socialist parties because, as most of the Brothers are in the low income bracket, any direct and immediate financial support (as typically offered by these parties) would be highly desirable. In a similar vein, Witnesses are typically the kind of people who want instant help, rather than a people who would be happy to the government's long term strategic planing projects to help the economy, like the investing in ports, bridges or cleaning up the environment.
Thus ... not right, not center, not green ... but left political parties, would be my guess here.