I figure that a debut album is just that: the first album released by a musical act. I don't care if Ronnie James Dio released a ton of albums before going solo. His first album is still the debut Dio album, and that's what matters to me. By the way, both that album and the debut Rainbow album own most other debuts. Here's a list of my other picks:
Angra--Angels Cry (One of the most unique progressive/power metal debuts in history)
Ark--Ark (One of the quirkiest, most intelligent, and most stunning debuts ever)
Circus Maximus--The 1st Chapter (Some of the best songwriting I've ever heard on a debut)
Crimson Glory--Crimson Glory (A true 80s gem)
Dream Theater--When Dream And Day Unite (I know that this one is always up for debate, but if you look past the production, you'd be hard pressed to find better songwriting and musicianship on a debut album)
Firehouse--Firehouse (Say what you want, but you'll never hear a better hard rock debut anywhere. The musicianship and songwriting is some of the best you'll hear in the genre, and the vocal harmonies are out of this world)
Marillion--Script For A Jester's Tear (This is the kind of material you would expect to hear from a band that's been releasing a string of increasingly mature albums. To this day, it's my favorite Fish-era album)
Stay metal. Never rust.
Albert