hmm...I think for starters, every subject is extremely different for each person and you must recognise this. IMO though the best and most enriching subjects would be the extensions whether it be for English, History, Math, Languages, VET or the NAS Visual Arts extension. This is simply because they focus and extend your natural talents in a way that the 2u courses wouldn't, and apart from the VET and NAS ones, they all make hefty UAI contributions. What you choose of course depends on the student and their interests and ambitions, for instance a humanities geared person may benefit from a combo like: 4u English, a performing arts subject, a history subject and a social science subject including an extension in another subject. Or a more rounded person may do say...English, Math, a language, a science, a social science or history subject with one or two extensions included while the more scientific person may go for English, 4u maths, and either 6u of science or 4u of science with a computing or engineering subject...and they're all good combos.
EDIT: I would like to add something since I am in a candid, and crabby mood - physics sucks, well and truly sucks as it is the watered down love child between ext math and chemistry with none of the enjoyable bits of its "parents". I also think VET subjects are absolutely yucky as is business and the computing subjects. I also loved bio,*loved* english, and have found some enjoyment in maths and chemistry,I also would have loved to do Legal, Ancient +Ext History, Drama or another language. That said, the bits in this edit may not apply to you in any way, shape or form so meh.