Lots of choices there.
For a keychain, you want light weight. That means a single blade with zytel/frn handles. Light and strong, but easy to open in an emergency when panic may cause some bumbling around. Loss of fine motor functions.
Since 1980, one of the most popular, light, rugged, economicly priced knives on the market has been the Gerber LST. At 1.5 onces, and a 3 1/2 inch closed length, it will fit on a keychain. Very easy to open. The LST junior is a bit smaller, and theres a micro LST that is only about 2 inches long closed. They will cut a seatbelt easy. Contrary to what some knife companys will have you believe, any sharp knife will cut a nylon seat belt.
The Gerber LST's I have had experiance with have been very rugged little tools for thier size and weight. They go for all of 17.95 from Smoky Mountain Knife works, and are still made in Portland Oregon, U.S.A. Comes with lifetime warentee.