When I decided to buy a Mac, I decided that there were only two apps I was willing to spend significant coin on - Photoshop was one and Microsoft Office was the other. Like it or not, Microsoft Office is the golden standard for office apps. If you need to share files with others, it is really the only choice.
If you are just looking for a good set of apps for your self, you may find NeoOffice to be the better choice - it is quite MS Office compatible and support excellent clones for Word, Powerpoint and Excel.
As was also pointed out above, Abiword is an excellent word processor and gnumeric is an excellent spreadsheet.
So, lots of options, but Microsoft Office is your best bet. Get the Student/Teacher version, only about $130.00.