Shubb C1 Standard Capo for Steel-String and Electric Guitars
For me, on a steel string acoustic with 0.11 strings, the Shubb C1 was the best. Its practical, really small, works great and doesn't adversely affect the sound.
It can be a little fiddly for a few minutes when tweaking it for the first time, but then never needs adjusting again. It requires both hands free to put it on, which might deter some players.
I love it and highly recommend it.
Ordered from Box Music and was sent swiftly and well packaged.