2 greatest? McGrath and... surely not Gillespie?
Ahem... second greatest. Someone's forgetting Murali.
Not denying he's talented, but his head doesn't seem to be in the right place. Seems to be coming down with a bad case of the Ian Bells, mentally fragile. I'd say Australia's two best players so far have been Hussey and Haddin, both experienced players.
Don't really see what youngsters the Aussies have that are better than their current players (with the exception of the promising aforementioned Smith). I mean, their backup for Bollinger? Hilfenhaus. Doherty? Probably Smith, but seeing as they seemingly don't want to play him, Hauritz. Top-order batsmen? Hughes is young and talented, but as I said mentally fragile.
See now much as I like Brezza, he has no x-factor. He's a workhorse, a good batter and fielder, and able to tie up an end with reasonably quick, swingy, accurate bowling. Tremlett on the other hand is both tall AND fast, and can be a lethal player on his day (i.e. when he's not injured). To use a faintly stupid 2005-era example, if Broad is McGrath, a seamer with vicious accuracy, Tremlett is Gillespie, taller, faster, less accurate but whilst not as good a player, he's dangerous. Bresnan is Kasprowicz: he's average, can tie up an end, fairly accurate.