Tbf he's also the only Q6 that could have pulled off the victory against Kochou.
It's impossible for us to know that, really.
While I personally regard Kan Ki's victory over Ko Chou as the #1 or #2 feat of generalship demonstrated in the manga thus far (Ou Sen's overall WZI victory being the other contender), I think it's important to keep in mind that, as impressive as Kan Ki taking on 240K with just 80K at 3:1 odds was, we've seen similar feats before:
- Yo Tan Wa defeated a 171K army at close to 3:1 odds with just 60K starving troops,
- Duke Hyou held back an invading 100K army led by Go Hou Mei with just 10K soldiers for 4 days at 10:1 odds
- Tou held back a 50K Chu army around the same time and stalled them time with just 5K men, also at 10:1 odds
Kan Ki did his business, but it can't be understated how much heavy lifting 6GG talents like Shin, Ou Hon, Kyou Kai, Ka Ryo Ten and others such as A Ka Kin and Kan Jou contributed through delivering on such a crucial victory at Eikyuu (and none of the aforementioned feats involved that kind of firepower).
But to get back to the original point: I certainly disagree Kan Ki is the only one of the 6GG who could've won.
The way I see it, Kan Ki taking on Ko Chou in the manner and under the circumstances he did was a reflection of his character. To win that battle he took a meatgrinder approach by which he "buried many an enemy," though "many Qin livers were violently squandered".
It's not necessarily true none of the other 6GG could've won against Ko Chou. It
is likely true none of them would take a battle with those odds if they could avoid it because (among many other reasons) they would never squander the lives of their own troops in the manner Kan Ki did for many reasons, I suspect not the least of which being sustainability.