Although I wouldn't recommend it on a keychain, my favorite in this category is the A.G. Russell 3" midlock. Essentially the same knife is the G.Sakai Preppy. The AGR in in titanium would hold up fine on a keychain, but I'm less confident in the carbon fibre one. Flat as a pancake and much thinner than one, with an excellent blade to handle ratio of 2.5" to 3.0". Weighs about .6 oz. I pretty much carry it regardless of whatever other knife/knives I carry because it's so unobtrusive that there's no reason not to.
On my keychain I have a Spyderco Honeybee (the middle sized one of the lineup). Blade steel isn't much to brag about, but it's tiny and razor sharp. Great for splinters and other stuff where a needle tip is needed.