There are two modern choices: the Penguin translation by Rex Warner, and the Oxford World's Classics translation by Martin Hammond. They're both very good: Warner is perhaps a touch more readable, and has a companion commentary by David Cartwright, published by Michigan.

You may also hear the Landmark Thucydides recommended to you. It is lavishly ornamented, but uses a rather archaic translation, and is dishonest about the fact, pretending that it's a fresh one. I would not recommend it.

As a sample, here's the first sentence of the funeral oration (the opening of Thuc. 2.35) in each of these versions. The funeral oration is often regarded as one of the most difficult and convoluted bits of ancient Greek prose in existence (I'm not sure about that; some of the more abstract bits of Plato are monstrous, though that's more because of content than because of style).

Rex Warner, 1954 (Penguin edition):

Many of those who have spoken here in the past have praised the institution of this speech at the close of our ceremony. It seemed to them a mark of honour to our soldiers who have fallen in war that a speech should be made over them. I do not agree. These men have shown themselves valiant in action, and it would be enough, I think, for their glories to be proclaimed in action, as you have just seen it done at this funeral organized by the state. Our belief in the courage and manliness of so many should not be hazarded on the goodness or badness of one man's speech.

Martin Hammond, 2009 (Oxford World's Classics):

Most of those who have spoken here on previous occasions have commended the man who added this oration to the ceremony: it is right and proper, they have said, that there should be this address at the burial of those who died in our wars. To me it would seem enough that men who showed their courage in actions should have their tribute too expressed in actions, as you can see we have done in the arrangements for this state funeral; but the valour of these many should not depend for credence on the chance of one man's speech, who may speak well or badly.

Richard Crawley, 1874 (used uncredited by the Landmark Thucydides):

Most of my predecessors in this place have commended him who made this speech part of the law, telling us that it is well that it should be delivered at the burial of those who fall in battle. For myself, I should have thought that the worth which had displayed itself in deeds would be sufficiently rewarded by honors also shown by deeds; such as you now see in this funeral prepared at the people's cost. And I could have wished that the reputations of many brave men were not to be imperiled in the mouth of a single individual, to stand or fall according as he spoke well or ill.