In my opinion this all depends on the scenario.
Against an offensive team, Groudon is certainly the most likely to pull off a sweep, as it can set up in many scenarios with one mis prediction, and isn't too worried by Extremespeed.
But against a full stall team, Gira-o has the best chance to reliably sweep. It can set up on many, many support pokemon, and has a better chance against stall due to bulky arceus' prominence.
But because of Groudon's ability to run an effective lum berry set, and beat some stall checks with good hazard conditions, it is better overall. It's important to note that it will usually just damage a stall check beyond repair and then be statused or phazed, so in the given scenario Gira-o is better against stall.
Groudon is certainly more useful against balance and offense in this scenario, so overall its more likely.
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