Kaido is one of the best characters introduced post-TS. Well, he is not as good as an antagonist as Doffy, nobody is. He is not as great as a character as Katakuri, Then how many characters we got in One Piece as good as Kuri? But compare Kaido to most other antagonists, he is a pretty good character.
Let's start with strength. Its true that strength doesn't make a character great always. But for Kaido's character, strength is probably the most important part. Kaido has been introduced to the readers as the strongest for quite some time. He got all this ridiculous hype since the suicide chapter. If Kaido hadn't delivered by one shotting Luffy, or carrying Onigashima, you would be first to call him a failure and a joke.
He is unpredictable. He had forgiven shutenmaru who fought him, but killed Orochi at the first dissent. He can be whimsical drunk at times and can be pretty cruel during others. I find him intriguing in the sense that we don't really know his motivation. Why he tries to wage war? Why he wants to die? There are other things which makes Kaido different. He is not like Akainu who will play every underhanded tricks to win - he gave the scabbards and supernovas every chance to hit him. On the other hand, he is not like Mihawk/Zoro who believes in fighting fair as we saw against Oden. Its still early to judge how complex or unique Kaido is as things can change in the future. But for me, he is already way better than Big Mom.
