Pokemon ATT/DEF Tier List
This guy has made an interesting tier list for ATT/DEF.
https://www.reddit.com/r/TheSilphRoad/comments/5w8l5h/add_ikts_attacker_and_defender_tier_list/
I wouldn't say it's completely comprehensive, but reasonably accurate (kinda). What does everyone think?
I am still waiting for gamepress to release theirs as should be more comprehensive, including multiple move set rankings.

Answers
Attacking list:
I have Dragonite, Tyranitar and Machamp as Tier 1.
The strength of Tyranitar and Machamp against Blissey and Snorlax makes them on the same level than Dragonite.
For Tier 2 I also have Flareon, Vaporeon and Heracross. But the list misses Charizard (against Exeggcutor), Gengar (the fastest damage dealer), and Jolteon (the best against Gyarados)
Snorlax, Blissey and Exeggcutor can only be Tier 3 since the two first are so slow and the third one is never the best attacker (Dragonite is better against Vaporeon).
In defense, everybody agrees with Blissey Tier 1 and Snorlax Tier 2.
Then everything is so weak compared to Blissey and Snorlax that it does not matter. Just put any high CP pokemon.
Disagree with this list almost entirely (after Blissey and Snorlax).
Dragonite is no where close to the same tier as Snorlax. In fact, I'd say Dragonite is the same tier as Gyarados now (at least in my area). While Dragonite's double weakness to ice wasn't such a bad weakness before when Lapras was the only real threat (as in a pokemon that could take on more than just Dragonite while having ice moves, if your Dragonite forced them to include a Cloyster in the attack it was worthwhile), but now that Piloswine is one of the highest DPS attackers (with powder snow's insane EPS and Avalanche being an excellent charge move) and Swinubs being EVERYWHERE (I already have 3 Piloswines with this moveset, and my girlfriend has 2) Dragonite is next to worthless on defense. Had a gym with three consecutive 3000+ Dragonites in it, and we beat all 3 using half of our Piloswine's health in about 20 seconds without dodging.
I'll disagree with the posts here claiming Rhydon doesn't deserve to be there - he's not a great defender but he has high CP to sit in gyms so he needs to be on the list. Further, a single Rhydon isn't bad at all - forcing a Vaporeon in to the attacking team isn't a bad thing if it isn't very useful in the rest of the gym. The issue is when you get to 2+ Rhydons because then a single Vaporeon will take them all out easily.
My list (left to right in a tier is stronger):
Tier 0 - Blissey
Tier 1 - Snorlax
Tier 2 (viable CP while being not trivial to counter) - Vaporeon, Exeggutor, Tyrannitar, Espeon
Tier 3 (high CP but trivial to counter) - Gyarados, Dragonite, Rhydon
Tier 4 (great defenders but too low CP to be a threat) - Lapras, Slowbro, Slowking, Steelix
Tier 5 (usable CP if you don't have anything above) - Flareon, Jolteon, Alakazam, Golem
Honorable mentions - I haven't seen Heracross or Ampharos so I can't comment on them (I haven't seen Steelix either but it's easier to guess with him).
Any gym tier list that puts Lapras under Gyarados and/or Rhyon/Golem needs to be checked again, Lapras got nerfed, but it got a good moveset – FB/HP. People uses Arcanine against Lapras is weak, and Omastar no longer gains a type advantage. It seems like Niantic plays a trick on it – making movesets better at the same time lowering its max CP.
Lapras takes longer to prestige against than Gyarados/Rhydon.
And since it has low CP, it will be at the bottom of the gym, meaning that you must prestige against it every time while an attacker will only need to attack it once to put it out of the gym.
So adding a Lapras over a Rhydon as the 10th pokemeon? Why not (good for your team, not so good for you). But apart from that, it is better for you and your team to put a Rhydon.
Great defensive list.
And great idea to split Tier 2, Tier 3 and Tier 4 in three different buckets.
Tier 3 are easier to counter but easier to prestige against. Not a bad choice to use Tier 3 Defender in a newly conquered gym: your team mate will easily prestige the gym making it stronger. While when the gym is higher, attacker will need to switch attacker if they really want to abuse their weakness.
That is why I usually put your tier 2 and tier 3 list as a single list.