3D applications all have their strengths and weaknesses. Depending on what you want to achieve there are plenty of options for the Mac.
dvGarage and their package that includes
Electric Image seems to be a 3d beginning or intermediate artists dream. They show you how to begin 'thinking in 3d'. The rendering is excellent in the program. This program can take some learning to master.
FormZ is another great package. I think it excels at modeling 3d objects.
Amapi 3d may be a bit unconventional, but models organic shapes quite differently from other applications and in a more intuitive fashion than some of the higher-end modeling applications. Also on their site is Carrara, a relatively new app that attempts to put a workflow into the application to help beginners understand the process of creating 3d animations and images.
Bryce handles creating landscapes and compositing 3d objects quite well. Then there are some such as Infini-D which you'll find on the same link as Bryce that seem more approachable for a beginner, but lack some of the key feature to producing true photorealistic rendering. Raydream Designer is another older 3d app.
Cinema4D seems to handle illustrations well and is a lilttle weaker than Electric Image or some others for the animation aspects.
Maya is the base application that is used in most movie effects studios today. It is just becoming available for MacOSX and should be a boon for doing animation on the Mac. It is a hugely expensive program, but does everything really well and takes years to master.
Lightwave is a venerable 3d app that began with the Amiga and has matured into a great app for animation and modeling. I haven't used it, but some people swear it is as good as more expensive apps.
StrataStudioPro is a good balance between cost and performance and results. They have a free downloadable version of their software (minus some features, but useful for learning and many uses)
Then there's Martin Hash's
Animation Master which is a remarkable program but limited in its import and exportability which several other apps are also guilty of.
And I'm sure I'm leaving out a few that other may be able to mention, but I know there are a few others (including Adobe's 3d illustration program that is fairly basic called Dimensions and hasn't been updated in years)
In all there are many applications to choose from. I would suggest you check out the publishers websites and see which appeals to you the most. Until you actually try them, you may not know if they are best suited for your needs.
I've used Infini-D and found it lacking in features to make realistic imagery. Strata is a good balance but at the time I tried it lacked a few key features I was looking for. Cinema4d has all the features I liked and found it approachable, but the animation I've heard is not the most intuitive. I am really looking forward to see what Maya will do for the Mac platform and 3d on the Mac.
Good luck and enjoy!