If you want a modem under Windows and Windows only, then almost any choice is ok. Diamond, Lucent, I've had many, and I was never disappointed.
Now, if you want to run *other* operating systems, getting a US Robotics is well worth the money.
Why?
Cheap modems are "WinModems", where the driver (actually the CPU) does most of the work. Hardware modems, as most US Robotics, have most of their own logic.