My first app has been in the top 1000 of its category most of the time. Never made it to the top 200 yet (been to 203 and 205 - argh). That's with zero marketing budget, no external reviews (yet), and just keyword matches and the occasional social media hit. It's kind of blind luck on my part, but having an app that sits in an underserved niche is key. Getting good keywords was a big boost.

Google Play has been a waste for me. I even launched a free ad-supported version of the same app there and it gets less downloads a day than I sell on iOS or Nook. Yes - more people buy the same app on Nook than download it for free from Google Play. That shows how broken discovery is on Google Play. Well that and how small the Nook store inventory is.