Tobias VS Steven
To be quite honest, I think Steven with the level of strength that his Metagross has shown, would win, having being able to go on par with Alain's Mega Charizard X, which has proven time and time again to have an insane amount of strength, having being able to go head to head with Steven's own Metagross and take on E4 level opponents with some difficulty, and do 10 battles in a row(albeit this was done with healing items, which takes away a little bit from this feat). Plus, it was able to go hold its own against the Primals, one of which it had a severe type disadvantage against and the other having some very powerful Water moves up its sleeve and last longer than ME Special 3 Charizard X until Rayquaza showed up. Other than that, we haven't seen much of what it can do.
On the other hand, we've got Tobias...while I do believe that he is a formidable trainer in his own right and managed to catch Legendaries in Poke Balls, indicating he is either a strong trainer who found and caught these Pokemon on his own OR as an alternative he managed to befriend these Pokemon somehow. He has shown to be a bit of an arrogant trainer, and he has done nothing but unfairly nuke everyone else who are using regular Pokemon. Not all that impressive if you ask me since it's pretty unfair to them since these Legendaries are on a whole other level naturally in terms of power level, but he did sweep trainers, likely using only 1 move for each Pokemon they sent out. The only one that has actually managed to defeat a Legendary is Ash, who seemed to actually use some strategy and powered through, and he's the only one who has knocked out a single of Tobias' Pokemon, while everyone else got wrecked. We haven't seen what Tobias can do against more experienced trainers such as Cynthia or Steven or any other E4 level trainer, however.
Which is why I think that Steven would win, since even though he has yet to reveal any other Pokemon other than his Metagross, it's likely that they are on somewhat of a similar level considering he is a Champion, and Metagross has proven to be able to hold it's own against Primal Kyogre and Groudon, despite having a type disadvantage. who are stronger than many of the other Mythicals, so who can't say it can't hold it's own against Darkrai in particular? Darkrai may have Dark Void, yes, which can then deal damage through the Bad Dreams ability, but....that's it. Tobias relies too much on brute force and not enough on defense while Steven has plenty of experience in using offensive and defensive measures. Are you telling me that Darkrai can tank a Mega Metagross's Tough Claws boosted Meteor Mash?
this is likely a very weak argument, feel free to smash it to pieces