The more I look at each rank's description, the more I feel Dialga could slot into S-Rank. It pulls off both defensive sets and offensive sets with aplomb. The defensive sets has a good defensive typing with only 2 weaknesses, not weak to Dragon or Dark (which is a far cry compared to most anything else that can be considered defensive), and resists tons of stuff. It also has a good support movepool: Stealth Rock is great on Dialga as it can support the team without forcing them to run something like Groudon (interferes with weather other than Sun) or Forretress (gives up offensive momentum). It has both Roar and Dragon Tail which Giratina-A also has, except Dialga doesn't have a ton of debilitating weaknesses nor does it hit like a sissy. Arceus-Steel is likely to give Dialga the most competition due to great speed, Recover and a boosting move, but its STAB has poor coverage and has no immediate phazing move (Perish Song eh) while Dialga has Dragon STAB which pummels shit even if uninvested. Offensive sets though, oh boy. Not only do they have killer power and coverage (STAB Dragon + Fire + Fighting/Ground from base 150 SpA = gg), but they have great bulk and are super resistant to hazards, unlike some wallbreakers (Kyurem-W, Mewtwo to an extent?). It is slow though (though Scarf can fix that issue somewhat), but when it can take the majority of attacks lobbed its way Dialga usually comes out on top in the exchange. Even mixed sets can take an opponent by surprise, as Dialga has the power to Outrage special walls and has the, well, bulk to make use of Bulk Up to perform a bulky-boosting role, which can prove to be difficult for even Groudon to take down. I really don't see how a team can't get better without a Dialga in it if they use it right, as it is versatile enough to perform damn near any role you want it to and does it well, as it provides so much while costing very little (Fighting + Ground weakness is super easy to compensate). Defensive, Wallbreaker, Scarfer, Bulky-boosting, Mixed...if you don't call this a good glue Pokemon I don't know what is.