I have a very sweet edition of the work called The Landmark Thucydides, which is a hardcover edition that includes lots of nice maps (so you can find those obscure cities that he mentions) and some other information not normally included in a softcover. Solid translation, too. If you're ever going to buy an edition of his work, this is the one you want.
Here are two quotes that I have in my "favorite quotes" file:
"Men's indignation, it seems, is more excited by legal wrong than by violent wrong; the first looks like being cheated by an equal, the second like being compelled by a superior."
- Book 1
"I know that the spirit which inspires men while they are being persuaded to make war is not always retained in action, that as circumstances change, resolutions change".
- Book 1 (from Pericles' speech)