it depends on the piece. I've done a lot of work in oils, such as:
[link] & Acrylics.
[link]Although I love the look oils give, my favorite way to work is in Acrylic now- mostly because it dries faster and I can move fluidly from one piece to another without a lot of wait time.
I especially like to use Acrylic on Illustration board, like for this tree piece.
[link]What that combination lets me do is use the Acrylic almost like a watercolor- washes, translucency, and layers, and then I do the detail work with colored pencil. It gives me maximum control.
Cheers!