Classic talks about something that is good and has been created over a long period of time. It also means a perfect or typical example of something.
The phrase instant classic is a contradiction and shouldn’t be used.
Classical talks about anything of the civilisation and culture of ancient Greece or Rome. It is also used to talk about pre-19th century culture such as music or dance.
- These classic designs always look good.
(created over a long period of time) - The E-type Jaguar is a classic British sports car.
(perfect example of something) - He studied classical literature at university.
(civilisation and culture of ancient Greece or Rome) - Mozart is probably the best-known classical composer.
(pre-19th century culture)