Best forge? I'm pretty sure that question is pretty obvious. As much as they had made the Halo 4 forge a ton easier to use with the implementation of magnets, duplication tools, and the lock feature... It's still quite a pain to utilize at times. Mostly because the magnets will mingle with every other magnet on the map so it takes a bit to line up the pieces you want correctly, and every time you use the bumpers, they are a lot more sensitive causing the block you're trying to equalize to move up and down much more vigorously. This fact bringing up the lack of the fine editing tool when you zoom in on the element and while you move it, it will move very slowly, making it simple for construction of very difficult alignments. And of course... There is no forge without forge SPACE... Unfortunately the halo 4 areas were too small, terrain that is a bit too uneven and unproportional to make use of in most areas. Even though we were given Forge Island... It is still a beta element, as it has no terrain to work with, blatantly a flat land.
That is why my vote will be given to the Reach forge. The editing was not AS easy as Halo 4, but it was a ton more fun and genuine, the pieces were very nice looking with the forerunner chrome design that synergize with one another very well. The fine editing tool helps where magnets had in a way failed, especially with the size of the forge area we were given with Forge World. It was beautiful, aesthetic, full of nice terrain and topography, and overall a fantastic place to not only forge, but to just have fun dropping tanks on one another.