OSC has the biggest support base. ZEN & CREloaded are both boosted version, with strong communities. I cant comment on Cube, as I have never used it, but Pete is the man for that!
Joomla has a VERY big community!!
Have a look at
www.testacart.com, where you can see several open source systems. Megento shows the most promise and is possibly the most powerful, but is a bitch to set up. I was impressed with prestashop.