Here’s my first swatch from the Follow the Leader Aran Knit (FLAK) KAL. This is a selection of possible filler stitches.
Before washing:

And after washing and a light blocking:

The stitches I used were (bottom to top):
Moss stitch
Broken rib
Seed stitch
Pattern 4.19 from The Harmony Guide to Aran and Fair Isle Knitting
Double moss stitch
As you can see, the moss stitch had a real problem with biasing. The seed stitch was way too wide. The double moss was exactly on gauge and the broken rib and pattern 4.19 were very close. So far my favorite is Pattern 4.19. If I end up using it I will have to find out if it has an actual name.
Want to join the KAL? More information is
here. (Thanks Cindy for the cool button on my sidebar - and the idiot-proof directions for getting it there!)