I like the Hexus II from Topeak, enough that I now have 3, one for each saddlepack and one for my rackpack. But I went on functionality and didn't consider weight, so I also carry a Leatherman style pliers multitool in each as well. I've not had to use a chaintool for myself but have lent it out once and used it on behalf of others twice.
I also have puncture kits, tubes, tyre boots, powerlinks in 3 sizes, a bit of chain and cableties, amongst other stuff - I'm not the fastest around but I'm the sort of cyclist it can be handy to have passing by if you have a mechanical.