Legendary matchups for Groudon

The issue with Dialga is DPS vs TDO. Groudon has the best TDO no doubt, but Machamp has much higher DPS, to the point where a good Groudon only competes with Machamp in sunny weather. Add that to the fact that DPS is generally the top metric for raids, and there ya go.

Short of a change in moveset, Groudon will likely remain the TDO go-to for most raids with a ground weakness, though it will probably never be the DPS king short of a double weakness to exploit.

In the future, it will reign supreme with Nihilego as well, assuming it's a raid boss.

Groudon will win the TDO race considerably due to 4X resist thunder and higher natural bulk.

If you argued it’s the best overall counter you wouldn’t be wrong, but I figured most will go in with 6 Machamp (or 5+1 groudon) which is not wrong either.

Mind telling me what data you're inputting for Pokebattler? According to my screen, the best counters at level 40 with best friends and extreme weather still rank Machamp as #1 in both TTW and estimator. Whereas Groudon is ranked at #6 and #5 respectively.

Against future Gen 5 legendaries Groudon will deal super effective damage to Reshiram, Zekrom, Cobalion, and Terrakion. Other dragons will be faster against the two dragons, fire and fighting types will be faster against Cobalion, and a variety of psychic, steel, water, and grass types will be faster against Terrakion.

Agaisnt dialga, from what i've seen, aside form looking at TDO, groudon's never number one, as its TTW is so far behind machamp, the supreme and thus baseline counter.

Against Zekrom and reshiram, I'd go with dragons first, even if they have dragon moves, other dragons will still be faster and we will have dialga by then, against electric moves on zekrom there's Garchomp 6x resistance to electric with the double dragon DPS and the resistance to fire on reshiram

Terrakion, metagross handles It supremely unless it's given ground moves, in which case we have mewtwo Kyogre, machamp, PF venusaur, beloom and too many pokemon to name

Cobalion's weaknesses are identical to registeel's, (fighting adds no weakness to cobalion, only more neutral damaging types) and fire was supreme in all weather compared to ground

Groudon will need precipice blades to stand out in these match ups or it will just be a tank counter

If and when raikou and Heatran return, it will be supreme, but only those legendary match ups for now

Honestly, I don't see why anyone cares about having the pinnacle of the penultimate team for raids. You're gonna beat the stupid thing so stop worrying about that extra point of damage for each pokemon. Such a waste of resources.
It's the PVP arena where each individual pokemon is going to matter.

Overall ranking isn't the best indicator of ability to shortman a raid or ability to get maximum damage balls, so really I don't see why you'd ever look at it. TTW and estimator are the most relevant metrics on pokebattler.