If Sulu is honest, he didn't think much of Chekov the first time they met. He'd heard of him in passing – the wunderkind who started the academy at like fifteen and won some kind of award for computational physics – but in person he just seemed like, well, a kid. Nice enough, and clearly smart, but nothing extraordinary.
Then Chekov saved his life – and Kirk's – when no one else could, and Sulu started to realise exactly what he was dealing with.
He isn't someone who's particularly prone to self deprecation; he's good at what he does, he knows that, and he has the position to prove it. You don't get a place on the flagship by being stupid or bad at your job. But Chekov is something else. He's one of the smartest people Sulu's ever met, and Starfleet has a lot of very smart people. He can see connections between things that almost no one else can, do calculations in his head that Sulu would need half an hour and a computer for. It's impressive, and intimidating, and often leaves him feeling like he's struggling to keep up.
"Theory is not everything, though," Chekov tells him. "Equations are easy, yes, but the way you can slingshot perfectly around a planet without even trying, I could never do that."
It's a good point, and Sulu can't help but smile. "Your equations and my flying," he says, leaning over to bump Chekov's shoulder with his own. "We'll be unstoppable."
And they are.