Yo-Yo Ma and Carlos Prieto, cellos
João Luiz and Douglas Lora, the Brasil Guitar Duo
The Brasil Guitar Duo may be the best such duo around, I don't know of any other which is better.
Yo-Yo Ma is, well, what else can you say but Yo-Yo Ma, one of the most adventuresome of top name classical musicians who have vastly expanded the repertoire and the range of playing. Carlos Pieto is excellent, as well.
Leo Brouwer must rank as one of the most significant composers for guitar in the history of the instrument, his compositions have a place like that of Debussy or Bela Bartok for piano in both the repertoire of the instrument, its musical language and in its position in music, in general.