There are dozens of varieties of carrots and everyone has a different favorite. Mine is Danvers Half-Long. As far as certified organic seed goes, I don't go that far.
The best I tried is TOUCHON. It's an old variety, presumably heirloom, and it's widely available. If you have a 'Target' store near you, you can find it there. Their seeds are even organically grown! ($2.29/packet, if I recall correctly). Gardpro, zone 5b