I'm not a fan of putting too much extra on my keychain. I prefer to keep items loose in my pocket. With that in mind, I have EDC'ed a Redilite lighter and have recently swapped it out in favor of this butane model I got at Fry's.
Both are easy to use with the Redilite having the added bonus of retaining flint/wheel in case fuel is gone.
Piezo-start lighter and Redilite are 2nd and 3rd down respectively.