There’s more than one way to find a good airfare online, as the Kayak-SideStep merger shows. But what’s the best way? Is there one site that you can rely on to find the best fare for all your travel needs? Or do you need to visit them all to ensure you don’t miss anything?

I set out to compare fare-finding services and booking engines, including Kayak, SideStep, Mobissimo, Farecast, Priceline, Expedia and Hotwire. To do so, I chose three itineraries: a business trip to San Francisco, a trip from the U.S. to South America, and a one-way trip from India to the UK.

The winner? Mobissimo.

Itinerary 1: Business Trip from Denver to San Francisco

When I travel on business, the dates are usually inflexible, because I’m likely attending a conference or other scheduled meeting. I choose nonstop because business travel is hassle enough without lengthening the amount of time it takes to get there and back. But even though my primary concern is convenience, I’ll still have to justify my choice to whomever is footing my business bill. So cost matters.

I started with Kayak, which offered me a fare of $231 with one stop. But I wanted to go nonstop, and for that, it provided a fare of $333. I then used its “compare fares” feature to check on Expedia, Priceline and Hotwire. Nonstop fares on those sites were about the same as Kayak: right around $330.

Hotwire offered a “limited rate” fare of $219, but I wasn’t able to find out the carrier, the times, or whether or not I’d have to make stops — hardly ideal for the business traveler.

SideStep looked like it might have a winner for me, telling me that United had a nonstop flight available for $242. Clicking through, however, revealed that United’s nonstop fare was actually $333.

But then I checked with Mobissimo and Farecast, both of which found a nonstop flight with Frontier for $210! Wow! About a third less — Om would be pleased with my frugality.

The next time I make business travel plans, I’ll be sure to check those sites first — and maybe even go directly to the Frontier site myself. As useful as fare finders are, they’re even more useful if you combine them with some personal knowledge of which airlines most economically serve your favorite routes.

Winners: Mobissimo and Farecast.

Itinerary 2: Trip Home to Paraguay

My Paraguayan au pair is traveling home in February to visit her family before staying in the U.S. for an additional year. As a bonus for her hard work, we’ve offered to pay for her ticket, but obviously we’d like to keep the price low.

The dates for this trip are flexible, but the departure and arrival airports are not. I chose Feb. 15th-29th as the travel days, but with the hope that I could specify “flexible dates” and be offered cheaper days to travel.

Since Mobissimo and Farecast were so helpful with my business trip, I started with them. Mobissimo turned in a pretty good choice, of $1,155.69, and Farecast offered a fare of $1,303. Neither, however, gave me the option of classifying my dates as “flexible.”

Kayak and Priceline each found a fare of $1,140 with Brazil’s largest airline, TAM, while Expedia offered a TAM fare of $1,334. Hotwire doesn’t cover that itinerary.

I wanted to use flexible dates to see if I could find an even cheaper flight, but Expedia’s flexible dates checkbox only works with popular U.S. routes and anyway, it didn’t find the best fare for me in the first place. Kayak doesn’t offer a flexible dates capability. Kayak offers a flexible dates feature, but you need to sign up for a free account to access it.

Priceline made it easy to select alternate departure and return dates, though, so I ran the itinerary through again, shifting up by one day then shifting back by one day. I got the same result: $1,140 flying either United or US Air along with TAM to South America.

Winners: Priceline and Kayak. Close runner-up: Mobissimo.

Itinerary 3: One way from Bangalore to London.

For my third itinerary, I had to use my imagination. So I imagined someone beginning a new life in a different country, moving from India to the UK. Since Mobissimo did well for me with both my first two itineraries, I started with them, using a travel date of March 1st, 2008. It gave me a price of $457 through ebookers.com.

Priceline requires U.S. departure airports, so that was out. Hotwire doesn’t handle one-way tickets and only covers travel in the U.S. and Canada.

Kayak, SideStep, Farecast and Expedia all turned up fares around $460, with Farecast offering up the lowest option, at $455. But then I noticed that I could easily compare fares on Orbitz, Travelation and Cheap Tickets from the SideStep window.

It looked like I’d made a smart move. Using it, I discovered that Orbitz and Cheap Tickets each had available fares of $392, and Travelation had a $385 ticket for a flight on Gulf Air with just one stop! But when I looked closer I noticed that once you added on taxes, the price came up to $462 — almost exactly what I’d found before. It was a good reminder to always compare fares with taxes and additional charges added on.

Winners: Everyone except Priceline and Hotwire.

Overall winner: Mobissimo. It came within a few dollars of the low fare on every itinerary. If I were actually shopping for trips, though, I’d probably run the itinerary through all the sites, to make sure I didn’t miss any great fares.