it would be difficult to say that one particular opening is better than the others Craig, although there are some that would definitely fall into the weaker category...
The Blackmar-Diemer would probably fall into the weaker category, even though i often play it myself, that'll be why top players don't play it. The thing is, at my level, people make mistakes, and it's difficult to find the right defensive move every time when your opponent has lots of pieces pointing at your king.
The Spanish and the Queen's Gambit are both vast in terms of theory, as are the Sicilian and King's Indian, because those are the most played openings by top players. The Catalan is also very popular just now, so because the top players are all playing them, it would suggest to me that those are probably the strongest openings to play.
If you are learning openings just now, one little tip is to notice the type of endgame that comes about from those openings, you'll be amazed how similar the endgames can be. Watch out for themes and motifs, the likelihood of things like opposite coloured bishops, good knight vs bad bishop (and vice-versa), pawn structures etc.
Anyway, i've rambled on for long enough, and i'm sure your dad would be able to tell you more than me ;-)
i would say its between the spanish,italian,queens gambit,french.black diemer gambit and the sicilian