Calico, be careful or you might start a flame war.
Basically everyone will tell you to use whatever they are using.
I'll try to give you my view of the apps.
3DSMax - Positives: It has everything but the kitchen sink - very powerful. Dominant in the games industry.
Negatives: Sort of feels like your using twenty different apps, poorly designed. Expensive $3000+
Maya - Positives: I enjoyed using Maya, better design than Max, but still has just about everything. Dominant in the movies.
Negatives: Could have a more coherent design, expensive $2500+ ??
XSI - Positives: The best design of any 3d application I have ever used, great animation ability and works well with other apps. The $500 Foundation version is a steal.
Negatives: Lack of plugins, smaller user base, lack of tutorials, and the particle system isn't the best.
Blender - A poor man's 3dsMax, many features but feels like 50 different apps. It's Free!