I disagree with Tyranitar is "good in this new meta as a defender", Mewtwo has come to the game but Machamp, Heracross and Vape still exist (I think at this point everyone has one, or many), plus Mew2 can learn FB and with PC it's charged quickly, seriously, against Vape and Machamp Tyra is kind of a pitiful defender.
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Gym Defender Rankings
I pretty much agree with the pattern and placement relative to others.
I also tend not to use "supreme" counters anymore as all gyms are so easy with only 6 pokemon who are often Decayed.... I just sweep them with Dragonites/T-tars/whatever the AI gives me.
That said... Does the rankings have to be so S-A-B-Cish. It implies a lot closer ranking beween S and A/B then I think.
I'd keep almost everything but make mine say "S" then C, C-, D, D- tiers. Just to highlight the Gap between the S tier and rest.
There is simply a way larger gap between S and A then there is between A-B or B-C or C-D\
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About Steelix, I agree with it, it detains Dragonite and Mewtwo (partially), and Tyra isn't a good option against it, but it gets demolished by the legendary fire mons and Machamp.
About the question the difference between the A+ defenders and the lower ones is minimal, you can take down a Steelix with Crunch with Machamp carefree while taking down a Hypno or a Slowbro with Tyranitar without worrying even the SE charged moves, however the time they took to defeat is sometimes crucial, specially when you try to get the 50 coins
A great defender is a defender that can discourage attacks for some hours or at least that will make your opponent waste the most time, and blissey is the best one to do so, chansey and snorlax are good too, plus they only have one weakness, and a moveset that counter that weakness, rare pokemon tend to hold the gym longer, because everyone tends to feed them, so a porygon2 gets feeded more than a vaporeon
