Favorite Dii band.

I enjoy most of the stuff he did though not a big Elf fan. Favorite is Black Sabbath, those albums rival the ones with Ozzy imo and he and Iommi were great together.

Dii is good, but Dio is better.

His stints with Rainbow and Sabbath were great, but I prefer his solo stuff.

Elf is OK too, but they were not on the same level as his later bands.

I need to proof better.

Rainbow
Dio
Sabbath

Dio

He gave us at least 3 classics with each band:

Rainbow, Rising, and LLR&R
H&H, Mob Rules, Dehumanizer 
Holy Diver, TLiL, Dream Evil

For me those first two Black Sabbath albums he did, along with his first two Dio the band albums stand out to me as his best work.

His stuff with Rainbow and the later Black Sabbath and later Dio solo albums are still really great too.  I haven't heard enough of Elf to really have an opinion (other than I wasn't a huge fan).
The Ronnie Dio & The Prophet's or even The Elves is so stylistically removed from anything I like that I couldn't even listen to it (not a fan of 50's style Doo-Wop stuff or even the 60's pop stuff)

When Dio was on, they were great but each album had a few skip tracks imo. Those BS albums are good throughout.

Get outta here. Love Dio!!!! LOVE LOVE LOVE DIO!!!

Hard to pick a favorite.  I'd say the albums I listened to most were "Heaven & Hell" by B.S. and "On Stage" by Rainbow.  
But there have been many other great songs along the way, and plenty of skip tracks.