I only really know the Dio-era albums, and they're all fantastic. Rising is the most 'metal' of the three. The first one might be a little mellow for your tastes (it was for me when I first heard it, anyway). I say go in this progression:
Rising
Long Live Rock 'N' Roll
Ritchie Blackmore's Rainbow
I think there may be good material on the later albums, but I couldn't say where. I heard the song "Death Alley Driver" on TV once, and it was pretty badass.