I actually got to 24 with Rattatas and Pidgeys. First off, they're easy to catch, so going for that Great or Excellent throw is easy, meaning you can get 150-200XP per catch. Second, Pidgeys only require 12 candies to evolve, so pop that lucky egg and start evolving. You'll get 1000XP per evolution with the lucky egg active. I recommend saving at least 300 Pidgey candies with the equivalent number of Pidgeys to evolve. Also, save any evolutions of pokemon you haven't seen yet. Every time you register something new to the dex, it's 500XP, with the lucky egg in effect, it's 1000XP. I purposely don't evolve stuff until I'm ready to pop the lucky egg.