Square or rectangular cages are best as they provide the most usable space for the budgie and make arranging the perches in a suitable way easier. Budgies prefer being at the very top of their cage, tall cages ultimately provide a much smaller amount of space.
In a wide cage, your budgie will make use of the entire space as they fly horizontally (from left to right) not straight up and down. Flat top cages provide a good place for your budgie to play when he/she is outside of the cage; playgyms, stands and baths can be placed on top.
Always buy the largest cage you can afford as budgies need a lot of space to play and stretch their wings, especially if they have to be in their cage for most of the day. 18"x18"x18" is a good size for a single budgie. You can also read Minimum cage size for one budgie AND Cages: The good and bad
If you are going to get a 2nd budgie in the future considering a bigger cage would be wise. Do know that you will need to quarantine any added budgies so having a smaller cage just for quarantine is advised. In case one of your budgies get's sick this would also be ideal for a hospital cage to quarantine your sick budgie.
If you buy a cage that is smaller than this it will be difficult to fit an adequate amount of varied perches and toys in the cage and your budgie may become bored which could lead to health problems. When purchasing a budgie cage, make sure that the bar spacing is no more than 1/2 inch. Anything larger could be enough for your bird to get his/her head stuck (causing injury or even death) or even escape.