Which online travel agency site is the best?

There's Expedia, Orbitz, Travelocity, and a bunch of others. Is one of them cheaper than others, or are they all pretty much the same? If some are better than others, which ones are so that when planning trips, I can just search those few instead of a dozen different ones (which is when it starts turning into a headache).

i found when i was looking for my trip in april past that they were all pretty much the same prices... some have different hoilday packages but if one has a different package at least one other site would have it too and still be the same price.... generically same price (one or two $ difference sometimes) and i looked every day for a month so i could get best price... we ended up just sticking to the company with the best tv commerical theme song haha