Logic and Cubase would be the heavy hitters.
Heavy in features and also heavy in price - they tend to be spendy.
iMac CoreDuo comes with iLife06 which has Garageband.
Garageband looks deceptively simple but can do quite a lot.
What isn't commonly known is that once you grow beyond Garageband,
Logic Express reads Garageband sessions and allows you to use them in
that application so you don't lose your old music once additional functionality
happens down the road (i.e. better sounding software synthesizers).
I would bet Logic Pro also allows for this functionality as well.
Protools is fine for recording but honestly it lacks in the MIDI implementation.
It's popular in the industry but for really doing some amazing stuff you need
the bigger apps IMHO. But if you want to show kids what is used in "real" (read:
studios that are on the way out due to people having this kind of stuff in their
spare rooms and bedrooms) studios then that might be the way to go.
You can do surprisingly many things with Garageband - I find it very fast
for writing tunes without belaboring over the complexities of some of the
bigger more expensive applications.
See:
http://www.myspace.com/aliensporebomb
The track "Iterations of a Scene" was 80 tracks and done in Garageband
on a G5 DP 2.5.
"Life" was done in Cubase in its entirety on a G4/400.
"Galaxy" was started in Cubase and completed in Garageband
started on a G4/400 and finished on the G5 DP 2.5.
"Eta Carinae" was started in Cubase and completed in Deck II
pretty much entirely on the G4/400.
For the most part, Garageband has replaced DeckII as my "once the
synthesizers are done part, I need to overdub some audio bits now"
application.