What is a best way to get a 25% duty cycle square wave (5 V, 1 MHz) on a board?
If you're considering this: -


  Combine op amps to generator the square wave


Then, the LTC6992 from here appears to do what you want.




  I am trying to get two off-phase 25% duty cycle square waves(1MHz) to
  control a full bridge circuit.


You might be interested in adding this circuit to the output of the LTC6992 then: -



Picture from here and original from here.

I mention the above because if you need to avoid MOSFET shoot-through in your H bridge, these could be useful additions.