I like my Creative Grid 4 1/2 x 12 1/2 inch ruler a lot. I also use a Creative Grid 6 1/2 inch square and 12 1/2 inch square. I have a 6 1/2 x 24 inch and rarely use it.
A right angle is a 90 degree angle. You can make one with any square ruler. The 90 degrees is where the sides meet in the corner. I suspect most 90 degree rulers would actually have two 45 degree angles and a 90 degree but you might want to check the pattern to be sure. Just remember the triangle rule: the total all the angles must equal 180 degrees.