For the new system, I would recommend NEVER putting a Gyarados or Rhydon in a gym. I always regrettably placed them in gyms using the old system because it was based on high CP, not defensive stats. However, now that gym placement is not based on CP anymore (which is by far my favorite feature of the update so far), you can use pokemon that are amazing defenders, but have been long underestimated due to CP limitations. Umbreon, for example, excels at soaking up damage with its very high DEF stat and HP. His CP currently doesn't get much past 1600, but he is a MUCH better defender than a Gyarados or Rhydon. Umbreon is also a dark type, with few counters available (unless you've played the main series games to know that fighting could be used). Another good one is Steelix, which has the highest DEF stat available except for shuckle. Foretress, Wigglytuff, and Wobaffet can also now be great additions to gyms without CP limitations. Maybe the Battledex will also include new entries on some of these new rising defenders! If you have any other ideas of good defenders/strategies, be sure to share your knowledge here!