3com/USR Winmodem, Lucent/LT Winmodem = Good.
Anything with the words HCF, Connexant, Rockwell, PcTel, HSP, Motorola or SM56 = bad.
3com (and USR, same difference) windmodems and Lucent (also known as LT) windmodems are fine really, the others though, which usually run under 20 bucks, tend to have problems connecting, especially if there is line noise. I have noticed many Connexant (formerly Rockwell) chipset based modems have problems connecting to Cisco equipment such as as5300's.
For hardware based modems, Zoom or 3com/USR are nice.