I've always found that fusion is extremely affective. But you'd atleast want something from the radioactive section if you're doing it without cheaty elements.
If you want a tiny but extremely powerful bomb,the steps are
1)pause the game
2) fill the screen with neutrons
3)in the console, type !set x neut 250 and !set y neut 300
note:250 and 300 is the centre of the screen, you can use some other coordinates if you want.
4) !set temp neut 999999999.....
5)if you want more power, make a container out of a breakable material around the neutrons and fill it with deutirium.
6)!set life deut 30000
that should make a really powerful bomb.
one disadvantage of this bomb is that it explodes when the game is unpaused.
if this thred is necroed, i hope my advise helped somebody if not duliks11