(I didn't get online yesterday, so I am running behind.)
Anyway, I don't read many books in translation, so most of the ones I can think of are quite (i.e., at least 700 years) old. Of that set, I think Chretien is my favourite.
His Arthurian romances are accessible and interesting, and occasionally tinged with (often unintentional) humour. (My favourite is somewhere in Erec & Enide, when he says something like, "You may have wondered how she managed to travel so many miles in so short a time. The truth is that a mile was rather shorter back then.") It is an easy, enjoyable read, without the overwrought language and moralizations of other Arthurian romances.
In addition, when combined with the Vulgate Cycle, you will finally understand many more of the jokes in Monty Python and the Holy Grail. There actually was an "twa's only a scratch"-style black knight, in case you were wondering.