The default numbers in most typefaces are called lining figures and are designed to be used in columns so that we can add them up. The problem is that lining numbers stand out too much in a line of type.
In terms of bang for your buck, there's nothing like using old-style numerals for giving your type a more sophisticated look. Old style figures are as tall as the x-height of your type and so have a better type color in body text than full-height "lining figures." Figures 3, 4, 5, 7, and 9 have descenders; while 6 and 8 have ascenders. There are four styles of numerals:
Tabular Lining and Tabular Oldstyle for use in tables.
Proportional Lining for use in text set in all caps
Proportional Oldstyle for use in body text.