That has been debated to death before and it obviously depends on the writer and version of the characters, but in most cases neither are fully immovable or unstoppable. If they were, Juggernaut would probably just be redirected and keep going.

A little more practically speaking, Juggernaut wins easily:

• The Blob is a mutant who can basically lock himself to the ground with a short distance around him and become immovable in relation to the ground while being able to absorb most impacts to effectively eliminate the force experience by him and thus the ground.

• The Juggernaut is not a mutant, but an avatar of Cyttorak the Destroyer, the God/Demon ruler of the Crimson Cosmos dimension. He can tap into more and more of that power to the point where he can fight pretty angry Hulk on almost equal ground.

Even if you let Blob be immovable and able to ignore all kinetic force, he still has a big weakness. As demonstrated when he tried to go after one of the intelligent versions of Hulk. He realized he couldn’t move or hurt him, so he grabbed his stomach and started pulling. It quickly hurt so much that he instinctively stopped anchoring himself to the ground. At which point he basically became an invincible bouncing ball, which is how Hulk treated him. Until he sent him flying by using a big metal girder as a bat.