Where is the best spot for aerial view of the Palm Jumeirah island? Is it worth to go to the Atlantis Hotel for sightseeing? Is it better to take the monorail or just by taxi if 1-2 persons going?
Unless you are a guest at Atlantis all you can see is the lobby/shops unless you pay to go to Aquaventure or one of the other psid-for attractions/restaurants etc. the best view of the Jumeirah Palm is from The Observatory Bar &Grill on the 53rd floor at the Marriott Harbour Hotel
I agree the best aerial view is from Observatory bar and I wouldn't bother going to Atlantis unless you intend to go to the waterpark.
If you are going to the waterpark then I would take a taxi rather than the monorail - you would have to take a taxi to the monorail station anyway and so you may as well just stay in the taxi rather than get out and pay the monorail fare.
Third vote for Observatory, easily the best option. Take time to try out the nice food though, especially the spare ribs 😄
Sightseeing is a little limited at Atlantis and around the Palm in general. If you're looking for a collection of good photo stops, some of the tours may be a better option for convenience and practicality (ie transport, getting the better spots etc.)