I'm using photoshop CC 2015 and I can't select Paint bucket anymore, only this annoying Material Drop Tool is shown instead of bucket in the Tools.
You can change between tools like Paint Bucket and Material Drop tool by pressing SHIFT + G
(Sorry I can't comment on the post by Joe - 'In the upper right corner, switch from 3D workspace to essentials.')
I think this answer is most relevant because I had the same issue and SHIFT+G was not working. For some reason my workspace also shifted to 3D Workspace and despite using Photoshop for years (and knowing all the keyboard shortcuts), I could not get to the paint bucket Also click-and-hold on the Material Drop Bucket did not provide other options as expected (gradient, paint bucket, etc) - UNTIL I changed the workspace to Essentials. So thumbs up to Joe.
Edit Toolbar also helped to add the "Paint Bucket tool" - but it is not clear why Adobe chose this behavior.
I also had the same problem, been using it for years and shift-G didn't work. None of the answers on google fixed this (why is there always someone who doesn't know the answer who says look on google?). It was switching from 3D to essentials on the right upper pull-down which fixed it. Don't know how it got into 3D.