Why do we care about best defensive moveset?

dude, you are just too noob and lack of knowledge and playtime

go and take a gym where the defenders are snorlax with zh/body slam, blissey with zh/dg, vaporeons with aqua tail, tyranitar with iron tail/crunch, dragonites with dt/or

then go and take a gym where the defenders are snorlax with lick/hyperbeam, blissey with pound/hyperbeam, vaporeons with hydropump, tyranitars with fireblast and bite, dragonite with dragon breath and hyberbeam

is not about if its going to stop him or not because with enough pots and high cp pokemons you can take any gym, is about maximize the defensive potential of that tower especially if the majority of people in that tower are friends, the question how much pots and revives is going to take you or will your bag be empty before you take it down, or will give up, or the first snorlax will own three of your pokemons..etc

An entirely bad moveset gym wastes the attacker the least time and the least potions.

ZH/BS snorlax or DG Blisseys encourages me to tread carefully. Otherwise there's no stopping me from spamming the trigger, knowing that I only have to dodge the hyper beam.

Also, if I am taking down a level 10 gym, I can definitely take down a 10 lick/hyper beam snorlax gym. I can't say the same for 10 ZH/BS.

If you put your crappy Rhydon with high CP in a gym to get a high spot how am I going to prestige it? You know since it won't be at the bottom

The short answer is that some people actually want to be good at this game.

I just put whatever I have as a high CP mon in a stable gym normally.

Don't hate the player, hate the game design...

I don't have enough candies to get 20 Blisseys, Dragonites, Rhydon's or what not to keep evolving for best or worst movesets to help a gym.

Maxed 2 Blisseys with the wrong moves and that's it. I didn't "choose" to not put a ZH/DG Blissey, but it's because I don't have one to put down honestly...

Good gym prestigers and any high level players who reads this or reddit will know how to deal with Blissey, Snorlax or anything honestly.

Low CP Blisseys are bad, but if I had the time/potions, I'd prestige it too and just get less prestige per run.

Actually, the OP touched on something that has not been given enough attention: rapid gym building as a deterrence.

It's clear that having a gym full of mon with great movesets will deter attackers, but we are talking about only one instance of taking down a gym here.

In the real world, at least in my area, there are so many gyms containing loads of high-CP mon with mediocre movesets, but they have been staying at lv 10 for at least a few weeks already.

The reason is that during the preceding weeks, any time one got taken down, it would always go back to being a lv 10 gym the next day, again full of high-CP mon with mediocre movesets.

Clear territories have been established in my city exactly because of this reason. Any time a different team takes a gym, its trainers always get booted almost immediately, and the gym goes back to lv 10 by the next day. After some time, people stop trying and just let the gyms stay.

tl;dr Rapid gym building by putting mon with subpar movesets can be a valid deterring strategy over a period of time, providing there is enough teamwork to get gyms up to lv 10 quickly.