i agree with Hokai that we should be careful about talking about "mastery" and attributing it to just anyone. whenever we make judgements on other people we always bring elements of our own shadows and projection. but still, separating the wheat from the chaff is an important skill that each of us have to possess in order to make skillful discerning.
that said, i define "master" as someone who's able to articulate the dharma in a very clear and concise way while embodying it at the same time. also, since i'm making a first person judgment, they have to have influenced me in a very deep way with regards to my worldview and spiritual practice.
in that respect, here's my short list.
Ken Wilber - uber-philosopher and practitioner. his work is unparalleled when it comes to articulating and integrating the dharma with other vast areas of knowledge. he had the most impact on me when it comes to philosophy and spirituality. however, he doesn't explicitly teach any method of awakening. he teaches by example (e.g. his own life). his preference is pointing out instructions. he is a vast portal of Buddhist (and non-buddhist) dharma.
Shinzen Young - if Wilber integrates the philosophy side of the dharma, then Shinzen integrates the "technology" of the dharma. his TSoE (The Science of Enlightenment) is, for me, the best explanation/description of the path of awakening done in a secular language. he is a master of language and scientific metaphors for the dharma. plus, he seems to be an all-around nice guy. my kind of kick ass dharma teacher

Daniel Ingram - for kicking my butt with MCTB and reminding me the importance of practice, and for kicking off Dharma Overground.
if and when i awaken in this lifetime, i feel that the expression that would come through this bodymind would be a healthy one. thanks to those people i've mentioned above.
~C