You can't say really because we only know 2 of Tobias's Pokemon - Latios and Darkrai.
Similarly we only know 3 of Cynthia's Pokemon - Garchomp, Gastrodon and Glaceon (Lol all starting with G xD) and only Garchomp and Gastrodon have been seen battling.
Tobias swept through every gym with his Darkrai , and Ash was the only trainer to have defeated it with Sceptile, as well as defeating Latios (in a tie) with Pikachu. We don't know, but there is a chance he did it to the Sunyshore Gym & Volkner, who is considered the strongest of the Sinnoh Gym Leaders, and a match even for one of the Elite 4 like Flint.
However Cynthia is the 'undisputed champion' of the Sinnoh Region, and her Garchomp swept through Paul's entire team when he challanged her, and Paul is shown to be on a similar level to Ash. Also Cynthia is shown defeating various strong trainers such as Aaron, the bug type specialist in the Elite 4 and Flint, the fire type specialist. Also Garchomp has never shown to be defeated yet in the Anime, and Cynthia was confident enough to pit it up against attacks from Dialga/Palkia.
So yeah it's really hard to say :P but It would probably be the match of the Century. They should make an episode on it! :D