... Force-user would win, especially Dark Side user - they can absorb energy and redirect it.
Whoever tries to absorb the energy of a BFG9000 blast would probably go supernova right about then...

Anyway, I'd trade my BFG9000 for a Tardis and go pursue some more peaceful endeavors if the lightsaber wielder would switch to the light side of the force...

EDIT: but just in case they really push me, I always carry my experimental "planet buster" BFG10K in the backpack...

Maybe a collection of Force-users, set up around a Massassai temple to focus the energy centrally ... yeah, that might work.
Also, BFG10K is a rapid-fire, localised plasma-stream - an overpowered plasma rifle basically.