What is the most efficient way to initialize a Class in Ruby with different parameters and default values?

The typical way to solve this problem is with a hash that has a default value. Ruby has a nice syntax for passing hash values, if the hash is the last parameter to a method.

class Fruit
  attr_accessor :color, :type

  def initialize(params = {})
    @color = params.fetch(:color, 'green')
    @type = params.fetch(:type, 'pear')
  end

  def to_s
    "#{color} #{type}"
  end
end

puts(Fruit.new)                                    # prints: green pear
puts(Fruit.new(:color => 'red', :type => 'grape')) # prints: red grape
puts(Fruit.new(:type => 'pomegranate')) # prints: green pomegranate

A good overview is here: http://deepfall.blogspot.com/2008/08/named-parameters-in-ruby.html