Obviously you want the highest quality switch you can get for a kill switch, so it doesn't accidentally shut the car off while you are driving.

Are there any popular switches that people here recommend?

I know that those external kill switches required for most forms of wheel to wheel racing are high quality, except they are huge!

What else have people been using?

 

You dont want to use a "Switch" for a kill switch. You want to use a switch to drive a relay for a kill switch.

When shopping for switched you should be able to get a data sheet which lists the MTBF.
MTBF= Mean Time Before Failure. The longer MTBF the longer the switch "should" last. But keep in mind higher current levels will reduce MTBF. Use the switch to drive a relay which is made for switch large current levels. Search for a quality relay with a high MTBF. Personally I think the Honda "Main relay" makes an great relay to be used in extrenal application. Pop one out at your local junkyard. 100 times better then the relays you pick up at your local audio store.