Good defenders for high-density areas
The Gym Defenders Tier List is explicitly marked as outdated. So is the main Meta Analysis article, which was written before Gardevoir and Metagross were released.
In high-density areas, motivation decay is less relevant and Golden Razzberry feeding is prevalent. Expect players to bring top attackers (notably Machamp, Dragonite, Tyranitar and legendaries); do not expect psychological tricks to work.
Of course, the Normal-type tanks define the defensive metagame, but which defenders would you place after (or preferably between) them, to use up enemy time and potions/revives, force frequent switches and make Golden Razzberry feeding easier?
Candidates include:
1. Gardevoir with Confusion + Dazzling Gleam: Completely walls Machamp and Dragonite, with neutral matchups against Tyranitar and most legendaries.
2. Dragonite with Dragon Tail + Outrage: Walls Machamp, stops Entei/Moltres and hurts Dragonite badly. Optimal counters are more specialised but make berry feeding unviable.
3. Slowbro with Confusion + Ice Beam: Walls Machamp, punishes Dragonite and stops Entei/Moltres, but weak to Tyranitar and Raikou.
4. Gyarados with Waterfall + Crunch: Stops Machamp, Mewtwo, Tyranitar and Entei/Moltres, but berry feeding is unviable due to a double weakness to Raikou/Zapdos.
5. Steelix with Dragon Tail + Crunch: Walls Dragonite, Mewtwo and even Raikou/Zapdos. Good for berry feeding due to its impressive bulk despite weakness to Machamp and Entei/Moltres.
6. Metagross with Zen Headbutt + Flash Cannon: Stops Machamp, Dragonite and Mewtwo, with the bulk for berry feeding, but weak to Tyranitar and Entei/Moltres.
7. Clefable with Zen Headbutt + Dazzling Gleam: Walls Machamp, Tyranitar and especially Dragonite. Neutral matchups with most legendaries. Optimal counters are specialised.
What about others, like Lapras/Walrein, Ampharos, Umbreon, Milotic and Hariyama?
Answers
Lapras was God in the old gym meta, but nowadays it's just an annoyance. I think the top non-blisslaxchan nowadays is definitely Gardevoir, and maybe Wigglituff. Those two cover all the main threats IMO. Well, at least for me. There's nothing that makes me sigh out loud more than seeing those two in gym. On the other hand, I love when other players put Dragonite, Gyarados or Ttar in gym since that means it will be much easier to take down. Steelix can be very annoying as well.
"People look at it and think it's great" there's one of your problems right there. Not all Trainers need to read or compare the numbers and resilience's between Wiggly and Clefable to determine who is better, no, people can come to their own conclusions via experience, too. Personally, I always suffer more damage from Wiggly because Play Rough is harder to dodge than Dazzling Gleam, therefore, I find Wiggly to be better than Clefable. It really is that simple.
... Play Rough is no joke, man. It hits hard and fast. I'm in a low turnover area so my boys and I lineup Blissey, Wiggly, Snorlax and Azumarill. If the wife's with us, she leaves a Chansey. I know it's a hard hitting lineup because my mates in Valor and Mystic copied and leave it for us to clear out 😂
I agree with most of your top candidates for exactly the same reasons.
Lapras is still great, it's quite slow to defeat even with Machamp due to its amazing bulk. Walrein is a budget Lapras for defending, less bulk so less good. Ampharos, Umbreon and Hariyama aren't bad. Milotic is very ok, its movesets are punishing and it has bulk.