I have this problem in my house as we speak! I've tackled it a lot in the past and it doesn't usually involve using rulers or brushes.
It really helps if you have another toilet and you can leave this one to "stew" for a while. The water will break down the blockage over time.
I like to add a LOT of bleach to the water - this aids in the breakdown too. I sometimes add detergent but not sure if this really helps or not.
Most effective is the use of PRESSURE. I usually achieve the additional pressure by filling the bowl as high as possible using an external source (to make sure it doesn't overflow). Usually the water will gradually trickle away, but you need to stand by and keep topping-up to keep the pressure as high as possible on the blockage.
A combination of these techniques nearly always prevails eventually.
UPDATE: After having typed above, went back to my toilet which had been left for about 3 hours. Filled it with water up to the brim and, like magic, the blockage swooshed away!