"fltplan.com ", because "flightplan" will take you to another site(for sure) that costs money(I think).
I began using fltplan a number of months ago and it's just awesome. Very little I would change about it. A friend's department recently switched to Airinc and it's supposed to be really really good. I was really impressed when I heard what they did for you, but it costs thousands a year(per a/c).
I haven't seen or heard of anything that tops fltplan. Especially for free.