Just saw a post pointing out that the new gym defender tier list is no longer new. The list could definitely use some update. Since I’m not the owner, I’ll instead share my personal gym defender lineup here (trade secret!). It extensively takes advantage of force switching.
The proposed lineup is suitable for highly competitive areas and/or pre-raid scenarios. It doesn’t consider 3-KO (three-room trick, sync attack, or whatever else you may call it) because no lineup really stands against that. I just assume all 6 defenders in your gym are feeding GRBs against multiple attackers.
The 1st slot is Blissey, with no doubt. Zen Headbutt + Dazzling Gleam. I noticed recently that some opened the 2nd move on Blissey. It’s not necessary but if you have done so, go with Psychic, because it hits faster.
The 2nd slot is Togekiss with Air Slash + Flamethrower + Aerial Ace. She serves as a “Machamp Asshole” and forces a switch. Flamethrower does very well against her counters including Steel-types and Ice-types. Dazzling Gleam could replace Aerial Ace; it hits harder but is less likely to fire when there are multiple attackers. Edit: Gardevoir also plays well in this slot and Confusion + Shadow Ball (anti M2 and Metagross) is the set to go.
The 3rd slot goes to Metagross. Zen Headbutt + Meteor Mash. Common counters to Togekiss frails before Metagross so it forces switch again.
The 4th slot goes to Milotic. Dragon Tail + Surf. Has to be Surf 'cause the other two won’t get fired when there are multiple players. It walls Fire-types/Ground-types that are used against the previous Metagross.
The 5th slot goes to Dragonite or Garchomp. Dragon Tail + Dragon Claw, Dragon Tail + Outrage. Some might disagree: that big fat dragon is doubly weak to Ice! Yes, but it’s after Milotic and no one will use Ice-type to fight Milotic, so it’s again a force switch. Salamence would do, too. Dragon Tail + Outrage/Hydro Pump. I’d actually prefer a pure-Dragon but there doesn’t seem to be a strong one.
The final slot is flexible because per my experiences, <50% attackers would actually fight all the way to this one. But if you want the strongest gym defending lineup, I’d recommend going with a bulky Steel-type like Melmetal. Slaking/Snorlax with Body Slam/Play Rough also fit in. Or something to brag like a shiny baby.
As a plus, the 2nd-6th slots can all be shiny. Go for it!