Lezyne Multi tool 12 stainless.
You can do virtually any repair or adjustment on a bike with it, including repairing a chain, or truing a wheel.
It's compact, light, is resistant to rust (which can be a problem if you leave a multi tool in your saddle bag over a period of years.) and has a nice little neoprene sleeve that fits over it, and protects against puncturing a tube in the saddle bag.