"No person could have been more nationalistic"

If someone had said that about you, then it simply means that this person has never met anyone more "nationalistic" than you.

"I couldn't have been better" means you are at your best or in other words, you (feel) better than ever before

No person could have been more nationalistic

If someone had said that about you, then it simply means that this person has never met anyone more "nationalistic" than you.

“I couldn't have been better” means you are at your best or in other words, you (feel) better than ever before.