I use the Aerosoft Airbus X. I love it and to me it seems quite realistic. I think they have released a extended version which has some more functions.
I also use some other programs that simulate weather, traffic and the scenery of the airport I'm flying into/out of. Addons are essential for a good simulator experience.
I advice you to look on YouTube for the aerosoft one, just type in "FSX aerosoft a320 review". There is tons of reviews and helpful info on it.