WHMCS is what we use here, but I was just turned on to Blesta and I like the direction of the software. Probably not use it myself as it would mean another change to our customers (last change was from MB a year ago I think).
You moved away from Modernbill too. I still have a v4 license before everything went screwed up. I vote for WHMCS. I have been trying to get the Blesta billing software to work. Maybe that is why Concor got turned on to it.