Weapons grade Uranium!! So easy a cave man could do it.
I think that the plutonium bomb is the easy one.
IIRC, the first nuclear device was basically like a doughnut.
What made it go critical was a piece that was shoved into the center of the doughnut, and it reached critical mass.
This is a very, very simple design and does not require alot of high-tech switches, etc. If you had the uranium, a lathe, and a few other things, you could make one in your garage.