Depends on the platform.
With Intel chips, it is generally a good idea to stick with an Intel chipset. It's also a good idea to steer clear of Via chipsets on the Intel platform, for they do not have a license from Intel to design for the Pentium4 platform. SiS is a good chipset to go with for value, their 648 chipset is a great performer.
On the AMD side of things, Via is the usual choice with their constant releases of the KTxxx chipset line. However, there are other options. the AMD761 chipset was great for stability but nowadays its performance lacks compared to newer chipsets. SiS' 735 and 745 are a great value. ALi's MaGiK is underrated although outdated. Then you have nVidia with the nForce, which is great for stability and a rich feature set (onboard GF2MX, sound). And then there's the upcoming nForce2, which should be the final SocketA mobo chipset unless SiS plans to come out with the 746.