Where is the best place to find dimond in minecraft?

Diamond can be found at the very bottom of the world so you'll have to travel down and explore the various mines, caves, and ravines until you come across it a vein of diamonds.

The less ideal way to do it is to dig all the way down until you reach bedrock and then just digging forward in one direction until you find diamond. It may take some time and a few picks but it usually works for me.