It totally varies, depending on when you fly (time of year and day), and from where to where, and even how long of a duration. Sometimes there are killer limited deals, and sometimes there is just expensive shit.
I got tickets from seattle to london for about $200 cheaper than anything else i could find after days and days of searching on airtravels.net. Next day even their rates when up about $100/ticket.
But then needing a ticket from seattle to denver the best deal i found was using sidestep.com and tweaking my dates a bit.
Basically... there is not a single best place to buy tickets, especially regarding cost. If you're frugal, you're going to have to search your ass off.