High end users don't have much of a choice: Regarding performance there is no alternative for Intel's BX chipset. ALi, SiS and VIA do offer Slot 1 chipsets, but they are only able of beating the Intel chipsets by features. The VIA Apollo Pro chipset already offers Ultra DMA/66 and besides the option of clocking the main memory asynchronously (at the AGP clock speed), as we know it from their Socket 7 chipsets. Thanks to the much faster L2 cache of the Pentium II you won't lose much performance doing this. And of course you can still use 66 MHz SDRAM in a computer with 100 MHz FSB processor.
Anyway, as a matter of fact all power users have to go for a BX board. Let me give you some information on things which may become important for you when chosing a new motherboard.
What do you really need/want? All boards offer the common stuff: two IDE channels (Ultra DMA/33), floppy controller, two serial ports (16550), one bi-directional parallel port, two USB ports and the AGP slot. But as performance differences are minimal, every feature could become decisive when chosing a motherboard. Just take a look at the following list; maybe some feature could be important for you:
Of course there might be some more features, but this list is meant to show you how to differentiate from the various boards available. To get detailed information, please take a look at the manufacturer's website. The amount of information would blow up a motherboard review tremendously; besides the technical data is changed permanently by most manufacturers. New revisions usually profit from new features.