Im a little late in this, but I would just like to add my two cents to the thread.
IMO, unless you can find a hot FMJ (50 grain bullet at no less than 800 FPS from the gun), I would go with Speers 35 grain Gold Dots. They dont expand, penitrate fairly well, and the extra velocity and shape make it more likely to penitrate bone, especially the skull, at most angles. 35 grain JHPs typically come out of the barrel at around 1000 FPS from the Raven. I think Stephen A. Camp reported just under 1000 FPS with the XTP.
I would avoid most FMJ ammo, since I dont think they are loaded hot enough for penitration, and although lighter, a non expanding JHP would most likely out penitrate a underpowered heavy FMJ. Goldenloki.com used to have a section on the .25 Raven, and IIRC the mentioned that he only got 720 FPS with the WWB 50 grain FMJ load from his Raven.
Ive been interested in getting a Raven for a while now, and wouldnt mind hiding it somewhere around the house as a quick access gun. When I do, my choice of ammo will be the 35 grain Gold Dots.
I think someone mentioned Winchesters ammunition, which I think is called Powerpoint. It is very similar in design to Corbons PowRball, weighs 45 grains, and is loaded on the warm side. It sounds like a pretty impressive load to me, and is probably worth trying.