If, like me, you are unable to read solely the Greek text, then the next best thing is to use the Loeb edition. The English translation, made by C. F. Smith in 1919, is excellent, and it is useful to be able to refer to the Greek on the facing pages.
I would recommend reading this along with The Landmark Thucydides, which also contains a fine translation, excellent notes and timelines, and, perhaps best of all, detailed maps.
Lastly, it is illuminating to consult Hobbes's translation along the way, as it is perhaps primarily through Hobbes that Thucydides's account of the Peloponnesian War has had the greatest impact on English-speaking culture, as well as on Western political philosophy.