The kind of software that is easiest to use varies with the kind of building you are designing. Sketch up is easy to use, and you can open your 3-D files in other programs to get good renders. Form-Z is awesome when you are designing buildings that are orthogonal for the most part. Maya and Rhino are great if you like blobs, and I know that with Maya and Form-Z you can move your project between the files to get the best of both.
I've heard great things about 3D-Max (3DS?): It sounds like it combines the flexibility of Maya with the accuracy of Form Z. Art*lantis is great if you need it to look realistic. Most important is what you can do with the software. Each program has a slightly different layout and if you learn one program really well, you can do most of what you want in it. Sketch-up is the possible exception to this.
Remember that your walk throughs and renders need to support the argument and thesis of your project, so choose a program that will give the feel you want. It is better to have a single clear and comprehensive image than a walkthrough that does not adequately describe the project.