What Is the best method to create memorable online advertisements?

Get information about your audience detaily, go simple, make story, put some emotion in mix and never forget to keep brand in audience's mind. Many adverts are great in story telling - stay in memories for pretty long time, but alas many of them nobody can connect to a brand after some time. 

Depend on the target audience.. Different approach will have different results.

Insight playing an important role to determine what type of execution suitable to achieve the main objective.

Assuming the objective is to create a memorable online advertisement, but what is the product or service? what is the message need to be deliver?

I can say, interactive is one of the best other than playing with audience emotion.

other than that. aim to Go Viral to multiple the impact