The allen keys of Topeak 21 multitools are badly sized and difficult to use eg adjusting brakes if you have a rack. I tried it, but didnt like it.
Some of their penknife-style folding tools are better (The Alien?)

The jaws of Cooltool are pretty lose, but the multi-sized allen keys are cool. With a new Campy hub, you dont ever need a spanner.

All the chaintools are for emergency use only.

A mini-Leatherman (the one with pliers and a handle that folds in half) is perfect, but shops in the UK have stopped stocking it. Do Leatherman still produce their smallest useful tool ?

I find that a Cooltool and a Leatherman together get me out of most jams.