To me a yarn that felts well is one that:
1) feltes quickly
2) felts without stitches showing
3) creates a nice fabric.
I have not had good luck with Lopi. It takes 5 times through my machine for me to get rid of the stitch definition. In single ply yarns, I like Lamb's Pride worsted best and Manos (Manos would be my first choice if it weren't so pricey).
Here's a link to a summary on felting yarns
-- distilled from the Knitty Forum by "Bean Knitting".