People are always quick to say "Maya". Which is fine, but Maya is not consumer level priced nor consumer level usably-friendly.
What kind of budget do you have? Are you a student of any kind who qualifies for an academic discount? Are you a pro who is willing to invest the money or a dabbler who needs something simple?
There are quite a few good choices if you are willing to take the time to check them out. But ultimately your budget and needs will determine what works best for you.
Here are some choices across the budget and power scale... (and dontforget mentioned above items like Lightwave, Strata and Cinema 4D. If I am not mistaken, an older version of Strata 3D was offered free on a MacAddict CD years ago?)
New:
highend...
AliasWavefront's Maya Complete 6.5, $2000 USD
(student price, $300-400 USD)
AliasWavefront's Maya Ultimate 6.5, $6000 USD
(no student version)
medium:
Eovia's Amapi Designer, $500 USD
(student version, $90 USD)
Eovia's Amapi Pro, $800 USD
(student version, $150 USD)
Eovia's Carrara Pro, $550 USD
(student version, $150 USD)
low end:
Daz3D's Bryce 5.0, $80 USD
(student version, $60 USD)
Free or cheap:
Blender3D, $free.
But it's confusing and I think difficult to use. However it offers everything you could need if you are willing to spend the hours and hours it takes to learn it.
Then there are lots of older versions of powerful programs that are cheap to buy (but not necessarily OS X native or upgradable)... such as Ray Dream Designer, InfiniD, and older versions of above mentioned software.
Then there is also:
Swift 3D
Amorphium
Poser
etc
I'm a fan of Bryce for simpler stuff, I use it often and have done so since it's days as New Worlds Explorer. I never really had a use for any of the higher end powerful programs, as Bryce accomplished what I needed each time. I've only recently DLed and started using Maya Personal Learning Edition (which is quite nice and since its free, its a great way to learn).
Anyhoo I hope that something I mentioned helps you along your path to 3D.