I have (or have had) at least ten complete sets of the Bartok quartets on LP and CD in my possession down the years, and have no hesitation in saying that this Keller Quartet set is the one I would recommend first, especially to anyone new to the work. Whilst I would not want to be without the insights of the Tokyo, Vegh and various Juillard versions (the Takacs never excited me, I'm afraid) I find that the Kellers engage, move and stimulate me throughout the cycle with remarkable consistency.
Why? Well, I think it's partly to do with the authentic sprung quality this Budapest group give to Bartok's rhythms - nothing mechanical here - and partly to do with the first violinist (András Keller's) unfailingly precise intonation and beauty of sound. There is no ostentatious showiness about these performances: just wonderfully vibrant music making, clearly caught by the Erato microphones and now issued at an unmissable price. Don't hesitate!