For 35mm:

Velvia: For total over cast days, and very drab/flat lighting conditions. The pop it adds is
helpful, but otherwise, it can produce some very strange colors that never
appear in nature.

Elite Chrome 100 General use.

Elite Chrome 200: Star trails and astro work.

Kodak E100S: My main landscape film. Lower, more realistic colors, and easier to work
with.
Gold 100: General use print film. Available anywher, cheap!

For 4x5:

Velvia
E100S
Delta 100