I will be visiting Siem Reap for only one day, What is the thing that you cant miss while visiting Ankor.

I think you have answered your own question. With only one day, and what must you NOT miss. Surely it must be Angkor Wat!

Angkor What is a bar in Siem Reap. Cute name, but it's now what I'd call unmissable.

Angkor Wat is the largest of the many dozens of Khmer temples in the Siem Reap area. I also consider Bayon to be unmissable.

Seeing the temples is probably the number one reasson why people go there in the first place- and yes one day is enough for some folks.
For others they would spend 2-5 days so know thyself- how much temple time are you good for?

Angkor Wat, Bayon, Ta Prohm. (If you had more time I'd include Banteay Srei and Beng Mealea.)

most people knock off viewing for a few hours mid day because of the heat....it you did not you could see quite a few of the main places which are right outside of SR....you will need a driver and a guide to get the most out of it....you can't do it on your own in one day unless you just did AW which is the main temple but only tells part of the story

btw, its called angkor wat

Knowing the "Boss" I have to settle for viewing the sand sculpture of Angor Wat at the Grand Palace in BKK!

Go early and see the sun rise behind Ankor Wat.

You could visit Roluos Group in the morning and the Angkor Wat in the afternoon then continue to witness sunset at the top of Phnom Bakheng to have a spectacular view of Angkor Wat.