Again, show me the chapter that explicitly states that Sasuke is Gaara's strongest opponent, because I've seen the anime fight and read the chapters myself, and nowhere does Gaara say that Sasuke was his strongest opponent. I've also looked at the guidebooks too, and nowhere do they say that Sasuke was Gaara's strongest opponent either. The most they say is that Sasuke fought above par against Gaara's attacks, but not that he was stronger than any opponent he'd faced. There's no direct evidence that Sasuke is above the strength of 4th or 5th Gate Lee. Sasuke was only able to seriously damage Gaara with the Chidori, and he never showed that he did more damage to Gaara's sand armor than Lee did. I already explained that the panting is likely more psychological than actual exhaustion, and besides, this lasted for like a few seconds at most. Gaara didn't show any noticeable exhaustion after Sasuke's taijutsu attacks.
I'll also remind you that Sasuke hadn't fully adjusted to using that level of speed. Gaara wasn't the only one panting after those taijutsu attacks.
Sasuke was also panting and out of breath as well, while Lee never showed that kind of exhaustion when using his regular speed. So how would Sasuke be able to keep up in a battle of speed against Lee in the First Gate, whose speed is five times greater than his regular speed, while maintaining that same high level of speed himself? And how would Sasuke be able to protect himself against Lee's high-speed combo too? The only reason Gaara survived is because his sand armor was protecting him and he himself didn't have to take any of the attacks, not to mention the Sand Shield cushioning the blow at the last second. And as mentioned before, given Gaara's decision to replace himself with a Sand Clone, I think even the First Gate's Hidden Lotus would be enough to damage Sasuke. If Sasuke were to take the same attacks that Gaara did while Lee was in the 4th gate, there's no way he'd be able to defend himself and no way he'd be able to endure those blows, especially not the Reverse Lotus. Lee just blitzes Sasuke.
As if that wasn't enough, Gaara was in a complete state of disbelief at Lee's strength and speed,
going so far as to ask himself if Lee was even human. While he admitted that Sasuke was a worthy opponent, he was never shown to be in the same state of shock at his power that he was with Lee. That's more proof that Lee's strength with the 4th and 5th Gate is above Sasuke.
