Loop as in Looping/layering/delay or Loop as in true bypass/buffered bypass to take pedals out of your signal?
BTW - Pedal questions go in the Amp Lounge, there is more traffic there too.
If it's the first, I use a Digitech Jamman, not bad, but it's too focused on storing many loops, and not focused enough on live looping. The Akai headrush is good, the Line 6 DL-4 has lovers and haters and I've really wanted to try a Backline Engineering Riffbox.