I only have 2 rulers..one is the 6 x 24in that many people have already mentioned. i also have a 6 x 6in. These are the two rulers i first bought when I started quilting a couple of years ago and find them very useful and so far havent needed another ruler...I have been thinking about getting a bigger square one maybe a 12 x 12 but will need to look into that one before spending my money as things are very expensive over here as well.
The foot I seem to use the most is the 1/4 in foot with the guide and will probably use the darning foot once I start stippling and free machine quilting more ( have to work up the courage )
I also have a walking foot which i also use regularly..
have fun shopping...