Spivak: his prose in Calculus and in his 5-volume Differential Geometry (of which I have only skimmed the first one) is pure joy; his formal style in Calculus on Manifolds is nice.
Apostol: no-nonsense, to the point, and crystal clear.
Lang: always talks to you like an intellegent human being, precise, concise, and never hides the (sometimes ugly) truth. His books are so packed with information, you'll learn so much from every one of them.
Gelfand: Russian master; his book on Calculus on Variations is beautiful, and his Lectures on Linear Algebra is better every time you read it. Also to the point, clear proofs, and very logically written books.
Landau: another Russian; although he doesn't always treat the mathematics rigorously, he has given me much physical insight. The books Classical Field Theory and Mechanics almost make me cry, can physics be presented more beautifully and clearly?