I think I'm going to try to tackle that giant wall of text :P, HELP ME ARCEUS?
Then it appears that most of the people in the top 100 on Pokémon Showdown's Ubers ladder are not "decent to good players" then, since I managed to block Stealth Rock with Espeon in the vast majority of my matches on my way up that ladder (which has the Wifi Clause, by the way, making Espeon significantly easier to predict and play around than in the many other ladders on Pokémon Online that don't have Team Preview).
Firstly, Showdown's Uber ladder is a lot like PO's ladder, its not exactly "great". As trickroom said that rarely judges people unless they're abusing research week or that attempt to revamp the ubers ladder which ironically failed (idk it just died).
Lugia, on the other hand, can tank Thunders, Stone Edges, +1 super effective Judgments, +2 Shadow Forces, and manage a large number of absolutely ridiculous defensive feats which no other Pokémon could even dream of achieving, and really only fears Cloyster's Icicle Spear and Teravolt or Turboblaze-enhanced attacks. And if Lugia paralyzes Kyurem-W, it can even use Roost and tank its Choice Specs Ice Beams.
Actually this is a really wierd arguement, while Lugia can stomach most of these moves, its probably going to lose Multiscale upon switching into one, then take double the damage from next move due to scale being broken. If these are really all the moves your speaking of then Lugia should be overwhelmed by the pressure and be either maimed, fainted, or too exhausted to pull of another move forcing it to rest. (hang on a sec I'm getting mah calcs :P).
To give a better idea of the idea above take for example a Lugia switching into the standard Scarf Kyogre Water Spout: (37.01 - 43.99%).
This may sound pitiful at first but the turn after... (73.79 - 87.25%), it will have a high chance to die this turn, so basically IT CANNOT SWITCH INTO EVERY MOVE :P. The calcs are similar with Thunder although the same thing basically happens, it can't switch in to it. Thunder will 2HKO after Multiscale is broken, although this is really unimportant. Lugia will be left with 25% HPish after the second thunder which means Lugia will be forced to either, Whirlwind or Paralyze Kyogre to avoid getting killed by it. Basically Lugia's inability to switch-in on most pokemon really render it hard to completely reap the benefits out of Multiscale.
Also this happens with almost every other poke too take Arceus Ghost here at +0 judging Lugia with Multiscale active: (30.76 - 36.53%)
Now it outspeeds and can hit it for massive damage right?: (61.05 - 72.59%)
Oh and wall breakers still break Lugia through Multiscale, for example... Specs Kyogre Water Spout vs your Lugia (55.52 - 65.62%). Yeah... its going to need help to wall the whole meta lol. Also chance are these calcs won't even matter due to Stealth Rock being up, speaking of which...
Excadrill and Swift Swim Pokémon need Sand Stream and Drizzle support respectively to be amazing, yet this does not stop the aforementioned from being considered so top tier in OU that this community has banned them from that metagame. Also, Lugia requiring Magic Bounce support is hardly a big deal when Magic Bounce support is one of the best things any team could have anyway (especially in the Übers tier, where Ferrothorn gets ample opportunities to set up Spikes due to the prevalence of rain, and where Rapid Spin is almost useless due to the existence of Arceus-Ghost and Giratina), due to its myriad of amazing utilities. Do not think of Magic Bounce support as being something that takes up an entire spot on a team just to prevent Stealth Rock from crippling Lugia. Rather, think of ALL the benefits Magic Bounce gives to the ENTIRE team - in my first post in this thread, I have listed nine wonders of Magic Bounce, and I would imagine that just about any team would kill to be able to reap the benefits of at least a few of them.
Before thinking of all the benefits Magic Bounce gives lets think of all the negative things it brings also. Lets see, your forced to use a Pokemon who is basically 2HKOed-OHKOed by the whole metagame unless there are screens up or your going against something incredible weak (referring to Xatu vs Ferro here, Espeon actually gets 2HKOed by Ferro Gyro ball). Your also forced to go mad with prediction if your using Espeon, its lessened with Xatu a little bit against Groudon and Ferro though. Regardless of what mon your using your either busting your brain trying to figure out how to predict whether a Dialga, Tyranitar, Froslass, Support Arceus, or Deoxys-a / n / s is going to use an offensive move or set up hazards. If you predict wrongly, you lose your magic bounce user. If you predict right, Tyranitar still check mates you with Pursuit, Deo-a / n / s can still smack you with a Shadow Ball, Dialga can end you with a Draco Meteor or Fire Blast, while Support Arceus can Judge you and you probably will die unless you manage to set up a screen before they move. If you switch out they can use Stealth Rocks. Think I'm giving the enemy a bit too much credit for prediction skills? I'm giving you enough credit for predicting right in the first place (tl;dr the prediction arguement works both ways, and we aren't even discussing the usefulness of mons with support really, lets put that to the side). I really don't think they deserve to be even tiered on to this list really, their true inferior offensive presence makes it look like they would really only deserve a slot on a support list (I really think we should make one of these :P). Also if we are going to attach Magic Bounce to Lugia what if we were to become magical prediction monsters (aka trickroom 9.9) and predict every move the opponent uses but use oh Giratina-a instead! Great now we have something that can wall the whole meta if it has dual screen support! Yeah I really don't think this list should factor THAT MUCH support at ALL. Because in that case, infiltrator Ninjask and Brick Break Gyrados would be OP and not tier 1 but tier -1 (if that makes any sense at all).
So in the end, I think we should all assume that tier one of these mons are really with little support to make official tiering (like little as in enough to become viable). Then everything else is listed as based on how much support it needs to reach the same level of usefulness that the tier 1 pokes can provide, (ex: Gastro needs the most support out of all mons to be viable which is why its so low) Lugia is not getting its own tier since it can't wall the whole meta, and like other walls, it can get broken by wall breakers, and hands down the ubers ladder cannot provide matches more epic then that of one against the fearsome trickroom. Also gyrados should really be #1, Taunt + DD is epic I tell you, sets up infront of so many stall mons..<=That is supposed to be invisible I don't get how people use that cross slash thing
Also Arceus-Ground needs to move up a tier, as Jibaku said, its offensive SD set can really lay havoc on enemy sun teams, and it doesn't need that much support at all.
If I sounded mean at all in this post I apologize :(.