Which travel company should Yahoo buy?

Yahoo has been on a shopping spree, and Tnooz is wondering which travel company the search giant should buy.

CEO Marissa Mayer is primarily interested in companies that make products that relate to users' "daily habits" -- a criterion that knocks many travel startups out of contention.

She also appears to want to improve Yahoo's expertise in its mobile offerings (especially in how its travel offering works on mobile devices) and to expand the company's reach geographically.

So far, frequent flier search startup MileWise is the only travel company Mayer has added to her basket.

Here are some candidates that travel industry insiders are proposing:

HotelTonightPros: No tricky back-end relationships to manage. Suppliers do the heavy lifting of loading inventory. Could extend the model of the last-minute mobile-first sale of hotel rooms to other categories. Cons: Might cost a pretty penny while antagonising larger ad buyers, such as Priceline.

HipmunkPros: Would immediately let Yahoo differentiate from Google in flight and hotel search while also strengthening the engineering team on Yahoo's travel site. Might be relatively cheap. Cons: Hipmunk needs help with monetization, and it's not clear anyone inside Yahoo would know how to add that missing ingredient.

TravelzooPros: It has 26 million email newsletter subscribers, hundreds of thousands of whom open its messages near-daily in markets outside of North America. It already runs a paid search strategy in partnership on Yahoo! Search. Yahoo could make travel ads richer, so they look alive and match the the dynamic content found elsewhere and expand the reach of ads beyond the newsletters, such as by pumping them in to a new travel a Tumblr-like feed or by serving the ads up in a locally relevant way to mobile users. Cons: TravelZoo appears to be a technologically backward company. Fighting with its corporate culture and outdated legacy systems could turn into a time sink.