Get Rainbow Rising and Mob Rules. NOW!
Then get Strange Highways and Operation: Mindcrime II.
Consider getting Magica, but understand it's a very slow, very cheesy album. I like Angry Machines, but nobody else does, so you might want to skip that. Intermisison is "meh" at best. Rainbow's Long Live Rock 'n' Roll is a good album, pick that up... but don't rush. Screw ALL early Dio before Elf. Forget you even know about it. Consider getting Elf, but... don't. Live Evil is worth $5 used if you ever find it.
12/7 - Jon Oliva