Uranium-235

U235 is a "rare" Uranium isotope. As rare things also happen to be the useful ones, U235 does have its uses over U238. It is quite infamous for being used in Little Boy and other nuclear fission weapons. Despite this, it is also useful in uranium fuel.

Uses
Yes, U235 can be used in nuclear warheads, specifically the "gun" type bombs.

It can also be used in uranium reactor fuel.

If you have no need for U235, you can breed it into Neptunium and breed that again to get Pu238.

It can be used in the Custom Nuke. It contributes a nice 15 to its nuclear stage.