What are the most important tools for a business analysis? How does MS Office support the job?

The most important tool is a tool that supports the modelling of business processes in UML, like ARIS.
MS Visio can do the job as well, but is in my opinion less suited for real complex processes.
I only use MS Office for supporting tasks, like Word for creating the supporting documentation, and Powerpoint to present the results to the management.

Top10 Business Analysis Tools/Products In Use (in order of frequency): MS Visio Enterprise Architect Rational Requisite Pro MS PowerPoint MS Word MS Excel DOORS Balsamiq (tie) Axure, BizAgi, Blueprint Requirements Center, CaseComplete, FreeMind, HP Quality Center, iRise, Mindjet MindManager, MS Access, MS OneNote, Visual Paradigm

MS Offices is only suitable for displaying information.   Excel can be a very usefull tool for calculating and analysing various aspects of the business unfortunatley it can only put out what you put in.......

MS Office is a power tool at initial stage particulaly during brain storming sessions.
Writing short notes, gathering functional procedure etc.
MS Word, Excel and Power Point play an important role in Requirement Gathering excercise  and presenations.
And then Visio will be a useful tool for creating good SRS. 

Excel integrated with Access are two very powerful tools to work with data.

You can use Power pivot wich is an excel add-in , it's very powerfull too.

Excel is powerful for calculation analysis and the built in "solve" add in is useful for optimization problems.

MS Offices is only suitable for displaying information.
It has no functionality nor ability to analyse a business in its own right.
However Excel can be a very usefull tool for calculating and analysing various aspects of the business unfortunatley it can only put out what you put in.
There a re many specific tools for monitoring a business however these tend to be either customised or very task focused.
  First thing you need to do is decide what information you need to analyse and then find the appropriate tool to carry out the task.
There is no real one fits all solution to business.

What about bizagi process modeler? I think it is a powerful modeling tool to analyze the process across the system, you can simply draw process flows for each functionality of the system you are trying to build or analyse in order to undestand the full system flow.
Welcome any new suggestion on this.
Thank you Regards Muzammil