I don't think I can pick favourites, I tend to really love everything... well, about a third from the huge playlist?
I enjoy everything by Wada Kouji, nearly everything by AiM, and Kazama Yuuto's pieces (Yamato) are also lovely. The evolution (and card slash) themes also make my ears very happy, and some of the anniversary/Christmas songs are gorgeous. Although not Masterpieces of Music, many of the other character image songs are wonderful, or at least highly amusing (Guilmon March, Agumon Ondo)... Never Give Up in Xros Wars and the various battle themes there are great, too!
.... I can see I'm not very good at answering your question. =w=;;