な adjectives have a property of being an adjective and a noun, and using の vs using な has a shift in meaning. When you use な, you're descibing the following noun. When you use の, you're basically categorizing the noun.
Good question. As far as I can tell, it’s an error. スーパー大辞典 lists 最高 as a noun. ウィズダム和英辞典 lists it as a noun and a の-adjective. JMDict has both.
In BCCWJ, there are 2410 examples of 最高の, and only less than 20 examples of 最高な + noun. As for the meaning, I see no difference. All the examples of 最高な + noun in BCCWJ seemed safely interchangeable with 最高の. stackexchange.com
I believe both are correct, but 最高の is overwhelmingly more common in general use. There's some cases where it wouldn't sound too weird like 最高な気分 (mentioned above) and 最高な一日 (first one that came to mind).
There IS a subtle nuance, but it's probably only perceivable by native speakers (allegedly).
