gonna necro this... because I can!!!!
Metagross
Metagross has definitely had one of the more storied careers in DOU, as throughout time it's established footholds in pretty much every DOU format, including the two older generations of ADV and DPP whose scenes only emerged quite recently. In the interest of discussing Metagross's eventual place in each metagame in a logically sequential manner, I'll be taking about the Generations in chronological order, rather than the order in which their DOU scenes emerged, to talk about what it's gained and lost throughout each turn of a Generation.
ADV and DPP both feature Metagross at, without a doubt, its best; currently standing as a Tier 1 option for both formats, Metagross stands as an immensely menacing attacker thanks to its staggering offensive presence and a defensive profile that makes it hard to deal with. While DPP's introduction of the physical/special split did give Metagross more offensive techs, both iterations of Metagross regardless play quite similarly as they aim to leverage their defensive Steel typing and high power to make most trades unfair. Access to a then unnerfed Explosion is perhaps one of the most impactful aspects of its kit, as this essentially lets players reset the board on their terms and forces opponents to play cautiously as long as they don't have lethal on Metagross to prevent Explosion; Metagross's ADV sets are essentially guaranteed to run the move, and, while DPP's increase in usable Steels / Ghosts have relegated it to being a mere option, it's still a viable option there to be respected for its game-deciding impact.
BW sees Metagross taking a step down from dominance, primarily because of metagame reasons—BW DOU simply has a plethora of top threats that make its bulk and defensive typing harder to leverage, as the metagame notably features strong Gem-boosted attacks, powerful Water-type-centric archetypes in rain and Jellicent SemiRooms, and more meta ways to hit Steel super effectively. In addition to this, Explosion also no longer halves the Defense of foes, making it significantly weaker and essentially making it unusable. Still, Metagross enjoys a top spot in BW despite the shift in playstyle; while it can no longer be played as an aggressive bruiser, it instead enjoys a place in the metagame as a centerpiece for bulkier offenses, being a threat that can systematically break down opposing teams behind a Substitute and with proper support. While its inherent characteristics are not as fundamentally strong in the BW metagame, it's still definitely strong enough to make it one of the better buildarounds the format has on offer.
XY is arguably its weakest Generation by modern standards. The loss of Steel's resistances to Dark and Ghost made it less valuable as a Pokemon overall, as a lot of its power lied in how difficult it was to break and it now has two new weaknesses, particularly to two key types as well with Knock Off being newly buffed and Mega Gengar being a notable XY terrorist. Despite the introduction of Metagross's Mega Evolution, the dominance of Mega Gengar and Volcanion simply makes Mega Metagross too much of a liability. Metagross's weakness in XY is historically glaring as, while it enjoys well-regarded positions in every other DOU format, XY is the one format in which it is not ranked in the Viability Rankings entirely. Still, Metagross did enjoy a career in XY while it was the current Generation; its base 110 Speed did let it outrun Mega Kangaskhan, its Steel-typing really handy for Sylveon, and its access to Ice Punch meant it was a physical attacker Landorus-T couldn't just simply beat (though almost all Lando-T in early- to mid-CG XY were Choice Scarf, anyway, with AV and Rocky Helmet being late XY / Modern XY developments). A more important fact, though, was that Volcanion only released later into XY's life as a CG, which meant people did not really adapt to it immediately nor did they play well with it, and people were also generally not as high into / good at playing with Mega Gengar at the time. To be clear, Mega Metagross still wasn't a top threat even at the time and was far from being one (and regular Metagross was essentially never used at all), but with XY being the one blemish of Metagross's career, it would fair to give it some credit and mention that it did still enjoy some relevance in the format for a period of time.
SM sees Metagross back to winning ways as it has now cemented itself as the undisuputed best Mega the format has, with some players of modern SM even calling for it to get banned. Mega Metagross in SM is definitely a much bigger beast than it was in XY—its base 110 Speed is immediately calculated and it now has access to actual Ground coverage. More importantly, however, its biggest competition is gone; both Mega Kangaskhan and Mega Gengar are banned from the format, which leaves Mega Metagross as the undisputed best Mega Evolution in the format. The banning of Mega Gengar is a big fundamental difference of SM from XY, in particular, as this meant that Mega Metagross will no longer easily get isolated and picked off. Now, Mega Metagross essentially represents what is essentially the biggest statstick SM has; with a menacing 145 Attack stat with Tough Claws-boosted attacks, Ice / Ground super effective coverage, Steel-typing backed by 80 / 150 / 110 bulk, and a relatively high base 110 Speed, Mega Metagross is essentially the closest you can get to a perfectly designed Pokemon with a neutral position in a metagame. While Mega Salamence shares the same BST, its stat-spread is not as efficient and being a Dragon / Flying type just gives it exploitable weaknesses to Ice and Fairy. Mega Metagross is just so generally usable in any SM archetype you can imagine that it is without a doubt a top 3 Pokemon in the format.
SS sees Metagross go back to its old ways with Mega Evolution not in the Generation, but this format is also proof that you
can teach an old dog new tricks. Back to its role as a tank 'n spank bruiser, this is the only format which features regular Metagross utilizing its now-Ghost- and Dark-weak typing primarily in a defensive manner. Despite this, Metagross currently stands as a viable utility pick in SS due to fulfilling various niches at the same time; its Steel / Psychic typing lets is answer a unique set of threats in Rillaboom, Diancie, Kyurem-B, and PsychicSpam, while its being a strong physical attacker with Clear Body lets it check CM Tapu Fini without fear of Intimidate. It could even set Stealth Rocks, and often did so for teams as its moveset was relatively lenient. The introduction of Steel Roller, in particular, revitalized Metagross as a Pokemon, as while removing Terrains doesn't actually matter in the grand scheme of things, Metagross being granted access to an accurate 130 BP STAB move does; despite the conditionality of Steel Roller as it requires a set Terrain to use, the move is still ultimately invaluable as it's essentially a Draco Meteor / Overheat without a stat reduction, which leaves Metagross still threatening with its other moves still at full power while also being able to use Steel Roller again without drawback if a Terrain ever gets set again (perhaps deliberately by its own trainer, as well). Though it is no longer the best of the best as it is now relegated to a utility pick compared to its days as an uncontestable bruiser, SS Metagross still enjoyed a history of success and had its own time in the limelight, its current Tier 3 ranking being very much well earned.
Throughout the many DOU Generations, Metagross has enjoyed a history of success. It is currently Tier 1 in ADV, DPP, and SM, Tier 2 in BW, and Tier 3 in SS—and while it is currently unranked in XY, it did still enjoy a period of success in the format, its Substitute set established by MajorBowman very much an icon of its time. It could be even fair to say that Mega Metagross is one ban away from being a top threat in XY, as while its design is uniquely positioned to take on the rest of the metagame, its lopsided matchup to one key threat in Mega Gengar is the one thing holding it back. Metagross's design of being a high-powered yet also defensively resilient bruiser is one that has stood the test of time, and its transformation into additionally being a blazing fast threat on top of that has lead it to having a dominant reputation in SM—overall, Metagross definitely has the right to being a Generational Titan of Doubles, being present in the Generation doubles was introduced to the franchise and having a history of success in every format thereafter.