It's hard to mention just one album. I use to name New England as one of my favourites, but I'd rather pick Twelve here, because it's just so breathtaking how musically adventurous he could be with just one single 12 string guitar and how he managed to capture the air of the twelve months.
But if I had to choose my favourite tracks from all over his albums, they would not be from Twelve but rather e. g. TGATG (2-pt title track), Tarka m. III, Sisters Of Remindum, La Dolorosa, Concerto De Alvarez, Tears On A Rainy Day and others (to name but a few).