Based on my gym experiences I have made a defender tier list. Assuming all pokemons are at level 30 and have equalish IVs.
Highest CP in each tier is the best defender due to gym placement, meaning that the highest CP within each tier is the best defender to use. Obviously higher tier is the first thing to consider when choosing defender.
Tier 1
Snorlax with Zen Headbutt and (Hyper Beam or Body Slam)
Tier 2
Dragonite with Steel Wing and Dragon Pulse
Snorlax with Lick and (Hyper Beam or Body Slam)
Snorlax with Zen Headbutt and Earthquake
Vaporeon with Water Gun and Water Pulse
Lapras with Ice Shard and (Ice Beam or Blizzard)
Tier 3
Dragonite with Steel Wing and (Hyper Beam or Dragon Pulse)
Dragonite with Dragon Breath and Dragon Pulse
Snorlax with Lick and Earthquake
Rhydon with Mud Slap and (Stone Edge or Earthquake)
Gyarados with Dragon Breath and/or Dragon Pulse
Vaporeon with Water Gun and Aqua Tail
Lapras with Frost Breath and Ice Beam
Lapras with Ice Shard and Dragon Pulse
Exeggutor with Confusion and (Psychic or Seed Bomb)
The rest is basically useless. The low CP but tough to beat (Omastar, Venusaur, Poliwrath, Slowbro, Wigglytuff, Kangaskhan, Chansey) simply do not work due to danger of losing spot and suboptimal prestiging.
Also, at this point, most people have enough level 30 pokemons from the listed tiers above.
Any thoughts?