After several years of trying out/maintaining a dozen or so different types of modems (I WANT ADSL...sigh), I've seen three stand out:
-USR ISA internal Courier v.Everything v.90 (the ones the other poster talked about.) These *are* the ultimate modems; they're rock solid even with lousy phone lines and have ping times as good as a modem can get in my experience. The external ones might be better, but you can't find those on E*ay for 30 bucks apiece in massive quantities
This is actually the reason my new motherboard is a KT7A- I wanted the ISA slot so I could keep mine...
-Diamond SupraExpress 56k Internal. The only one I used was extremely stable and had no problem whatsoever with bad phone lines or anything else.
-Lucent Winmodems. Included here because, if you have to have a Winmodem, it's probably the best choice; they don't break down much and the drivers are updated often. A couple of years ago, I ran one on a P133 that also had a 28.8 hardware modem installed; downloads notwithstanding, ping times were about equal, and there were no noticeable slowdowns (any more than usual ;P) while gaming.
Avoid:
-Diamond Supramax. Worst Winmodem, or any modem, I've ever heard of. Exception to the rule below...
-No-names. I'd rather have a USR Winmodem than an (insert third tier company here) hardware modem. Quality makes a difference.