Edith Grossman's is the hot new translation, but there may be a tendency to confer too much praise on a fresh reading. From what I have sampled, I have no doubt of Grossman's excellence, but this is not the "definitive" DQ (no one's is), and frankly, after some comparison of the early chapters, I've decided to spend my time with Burton Raffel's translation, now only a decade old. Raffel sometimes opts for a colloquial word or two, but it's never jarring, and his overall style seems not only less pretentious to me than Grossman's, but a superior combination of a modern reading with a traditional "tone." Tone and style are important, and Raffel sometimes makes Grossman seem too abstract or fussy, though this is difficult to describe. Raffel's phrasing is more focused and vigorous than Grossman's--though both are said to be accurate. Let me offer a couple of examples that shifted me toward Raffel:
"Some claim that his family name was Quixada, or Quexada, for there is a certain amount of disagreement among the authors who write of this matter, although reliable conjecture seems to indicate that his name was Quexana. But this does not matter very much to our story; in its telling there is absolutely no deviation from the truth."
"It's said his family name was Quijada, or maybe Quesada: there's some disagreement among the writers who've discussed the matter. But more than likely his name was really Quejana. Not that this makes much difference in our story; it's just important to tell things as faithfully as you can."
(Notice how Raffel makes immediately clear in the last sentence what Grossman so literally translates.)
"His fantasy filled with everything he had read in his books, enchantments as well as combats, battles, challenges, wounds, courtings, loves, torments, and other impossible foolishness, and he became so convinced in his imagination of the truth of all the countless grandiloquent and false inventions he read that for him no history in the world was truer. He would say that El Cid Ruy Diaz had been a very good knight but could not compare to Amadis, the Knight of the Blazing Sword, who with a single backstroke cut two ferocious and colossal giants in half."
"He filled his imagination full to bursting with everything he read in his books, from witchcraft to duels, battles, challenges, wounds, flirtations, love affairs, anguish, and impossible foolishness, packing it all so firmly into his head that these sensational schemes and dreams became the literal truth and, as far as he was concerned, there were no more certain histories anywhere on earth. He'd explain that Cid Ruy Diaz had been a very good knight, but simply couldn't be compared to the Knight of the Flaming Sword, who with one backhand stroke had cut in half two huge, fierce giants."
Notice that Grossman is rather fussy-sounding in the phrase: "countless grandiloquent and false inventions he read that for him no history in the world was truer." Compare with Raffel, who always seems to solve little problems like this with charm, precision, and even a little wry swagger that's so appropriate to Cervantes' intent. So my advice is to seek out both of these new translations and spend a little time with each before deciding. Don't take others' opinions that Grossman's has superseded Raffel's. Grossman avoids some of the more colloquial English one may find in Raffel, and this may please snobs, but the accuracy of Raffel's translation is not in question, and overall he seems to me to have done the best job.