I would say that the Mossie NF.30 was the best WW2 Night fighter as well. The He-219 would be in the same rank as well, however, they were never really given the chance to put these into full production/use. As for the P-61, it was also an excellent night fighter. I think, with the possible exception of the late mark NF. Mossies, that we must remember these aircraft were mainly designed to intercept bombers.
As for them fighting each other, it would partly depend on the tactics. I would have to say from a maneuverability standpoint, the win would go to the P-61. I remember reading of a dusk engagement between a P-61 and Ki-84, but I don't remember the outcome, except to say the P-61 didn't have a problem holding its own and RTB'ed.
However, from the interceptor standpoint, that would have to go to the Mossie NF.30 then the He-219, IMHO.