Nonsense. Shepard wasn't the only person leading a force of aliens to fight against Saren. What about Captain Kirrahe? He lead the assault on Saren's base of operations on Virmire and WON. If it wasn't for Kirrahe and his leadership skills, Saren would still have his base of operations on Virmire.
Kirrahe only wins with Shepard's assistance. Indeed, he only survives at all based on whether or not Shepard intervenes during the mission. That's a fact. If he and his men could have gone and done it themselves, they would have. STG aren't known for being a timid bunch. As it turned out, the Salarians acted as a distraction so Shepard could lead shadow team in the back door, knock out the AA guns and secure the bomb for detonation. Plus of course Shepard was the one that found the Virmire beacon and spoke directly with Sovereign. I doubt it was even aware of Kirrahe's existence.
As for the Asari and Turians, all they did at the battle of the Citadel was get their superior arses kicked up one side and down the other. It was the arrival of the human fleet that turned the tide and engage Sovereign directly after Shepard and her squad either Saren into breaking free of their control and committing suicide/beat him in a stand up fight then opened the arms to let the fleet in. Then of course Shepard and her team took down Sovereign's avatar, causing some weird feedback (still not sure that that works) that disabled the reaper long enough for Normandy and the fleet to deliver the killing blow.
Of course it's all irrelevant since these military victories aren't what proved or disproved humanity's "worthiness" to be turned into meat paste (a dubious honour at best), it was merely what caught the reapers' attention. The next two years spend abducting humans and testing the human genome's potential for mutation is what made them a "viable possibility
." Indeed, based on what Harbinger says, if the Krogan had not been sterilized, they might have been their prime candidates. Likewise, had the Quarians not had their immune system trashed, they might have been selected.
Again I say though, humans aren't "special", they're just well suited to the reapers' purposes. We can't even be sure exactly what the catalyst's criteria for preservation is. Either way, it's a subjective thing, it dose not confer some inherent worth above and beyond all others. If you need to put a nail inside a piece of wood, the best tool for the job is a hammer. That doesn't make the screwdriver a fundamentally inferior tool, yes?