We tested all the top players, on routes that run the gamut from simple to complex, to see where each type of site excels. Here's what we found.

It's no wonder that even the most skilled Web surfers find booking a flight overwhelming. First, there are the airlines' own sites; next, the major booking sites, such as Orbitz, Expedia, and Travelocity; and, finally, the aggregators (Mobissimo, Kayak, and Sidestep), which search all sorts of suppliers for deals. So where should you turn?

Difficulty Level 1: Seattle to Phoenix

June 16-23

Winners All the aggregators and booking sites found nonstop Alaska Airlines flights: on Mobissimo, Sidestep, and Alaska's own site prices started at $239; on Expedia, Travelocity, and Orbitz, at $244.
Losers United, by about $20— its one-stop service costs $260 round-trip.
Lessons Don't worry too much about relatively simple routes. If there's an obvious bargain out there, it will most likely appear with little price variation on most—if not all—aggregator and booking sites.

Difficulty Level 2: Los Angeles to Boston

June 16-23

Winners Sidestep, for its trio of offerings: it listed the cheapest round-trip route (two stops each way, on Midwest for $281), gave buyers the option of a great direct flight on Delta for $349, and offered nonstop service on JetBlue from Long Beach for $322.

Losers Expedia, because it listed Sidestep's $281 Midwest flight for $306 and didn't offer the runner-up for least expensive: a $300 one-stop flight via AirTran available on all the other booking and aggregator sites.

Lessons If you're flying on a route that JetBlue services (even through a secondary hub), turn to an aggregator. None of the major booking sites list any JetBlue flights.

Difficulty Level 3: San Francisco to Charleston

June 16-23

Winners Sidestep emerged victorious, locating the best values from both the San Francisco and nearby Oakland airports—$584 and $526, respectively. Sidestep and Kayak also listed several flights into Hilton Head, 60 miles from Charleston, for $200 less.

Losers Mobissimo listed numerous well-priced flight options, but these led mostly to cheaptickets. com itineraries that were no longer available. The first available flight was $592. Plus, the site didn't list any flights into alternate airports.

Lessons Choosing alternate airports can mean big savings. Most sites let you search multiple airports, but Sidestep and Kayak make it exceedingly easy, allowing you to quickly price out flights to and from other area hubs.

Difficulty Level 4: Atlanta to Buenos Aires

June 16-23

Winners Mobissimo found the best price, $990, with stops in Miami and São Paolo. Even better, the site found a $1,115 nonstop flight on Delta for just $125 more.

Losers Delta, for listing the same nonstop flight that Mobissimo found, at $226 higher. Travelocity also found that Delta flight—for an additional $230.

Lessons Although prices on aggregator and booking sites are, for most searches, relatively equal, with more complicated itineraries, there's always the chance of a cut-rate bargain showing up on just one site. If you don't want to miss out, check all the sites.

Difficulty Level 5: Dallas to Delhi

June 23-30

Winners Orbitz and Sidestep had the best mix of price and itinerary, with a combination British Airways and American Airlines flight for $1,476. For $500 more, American Airlines took nine and a half hours off the round-trip journey; Orbitz and Kayak also found this flight.

Losers Expedia, for flagging an American Airlines flight that was longer and more expensive than what American offered on its own site. Travelocity's most affordable option was a Continental flight for $1,904.

Lessons For complex, multiple-leg itineraries, booking sites like Orbitz are the best at putting together inexpensive flights using more than one carrier. The airlines' own sites sometimes lack this capability.

By Amy FarleyBridget Moriarity and Xander Kaplan