Hmm. Tough question. I own/have owned MM bags in Verona, Vachetta, velvet Verona, Nappa, Pompei, Toscania, Athene, Africa, Cuoio Toscano, goat (Sorrento), and pebbled. Truly, I love them all and for very different reasons, which is not a useful answer!
But...
if I have to choose, I would say that first, I love Marco's Nappa leather. I love shiny things and have traditionally loved smoother, supple leathers with a shine. The Nappa is such a wonderful leather choice for dressy or polished bags and while it has a glossy finish it also manages to feel soft. Every time I take my Black Nappa Modena out of its dustbag, my heart races a bit. Initially, the bag felt "hard" to me and not at all supple; but I have had the bag for almost two years now and the glossiness of it has mellowed as the bag has softened. It's perfect.
Over the past 5 years, however, I have also developed a soft spot for pebbled leathers. At first, it was because of their hardiness; they are forgiving, even if the pebbled texture does start to flatten out with use. But then I realized that I kept buying pebbled leather because of its dimensionality-- the pebbles create micro-shadows on a leather surface which makes this leather (and dye) look very animated and saturated to me. Marco's pebbled leather is calf and it is marvelous--soft and supple, with a lovely sheen that gets better over time. Some of my favorite colors in Marco's collection are pebbled leathers, and I think it is because of the depth of color that pebbled leather seems to have. I bet I would love Marco's metallic pebbled leather.
More recently, I have come to love natural vegetable-dyed leathers because of the artisanship involved in the processing of the leather and the variations this process produces. No two hides are ever alike. These leathers feel alive to me (as they once were, of course). I love the imperfections, the grain, the aging process, and the patina of these leathers. They take dye beautifully. I love these bags for travel, for weekends, or for workdays when I don't need a dressy-looking bag. That said, I did own a dark taupe Vachetta bag and that leather was smooth and perfect for dressier occasions, so I think it varies with Marco's leather. I will say I'm a bit more picky of when and how I want this type of leather on my bags because of their texture and variations. The Toscano leather has a nice mix of even dye and texture.