One question - hundreds of answers. It’s up to you, which poster is worth to buy. A good idea is to restrict yourself by choosing a theme.
You may collect posters by many parameters, please have a look at some examples:
Probably the most expansive manner to collect vintage airline posters. But nobody knows, how many copies of a vintage poster still exist. May be some early collectors saved their copy in the year of publishing...
And sometimes "younger" posters are much more expansive: TWA New York by David Klein (printed with fluorescent colors in 1958) is one of the most sought after modernist posters...
This is the most popular way to collect airline posters. In most countries exist(ed) just one major airline. May be it was a kind of national pride, to operate an ”own” airline.
Famous for their stunning posters are e.g. Air France, Pan American World Airways, TWA, United Air Lines, Swissair, BOAC and Qantas. Have a look at my collection!
May be you live in NY and need some decorative art for your loft? One of hundreds of reasons to collect posters of a continent, a country or a city.
By the way: does the Statue of Liberty raise the torch with the right or with the left hand?
This is for enthusiasts and technique freaks only. One of the most popular props in the fifties was the Lockheed Constellation (nickname "Connie"), a four-engined airliner with a triple-tail design and dolphin-shaped fuselage. The famous first double-deck aircraft (Boeing 377 Stratocruiser, nickname "Clipper") is illustrated on a lot of Pan American posters – highly collectable!
Some of the early poster designers are well known artists now. For example the famous art director Joseph Binder created a series of eight posters for United Air Lines. A very early example of corporate design.
By collecting posters of David Klein, (TWA), Bernard Villemot (Air France) or Donald Brun (Swissair) you can’t make big mistakes.
This is a little bit more difficult, there is no description on the poster which says this is "Art Deco" or "Modernism". If you are experienced in art, it is not a problem at all, but otherwise you need some help. Ask at a gallery, buy some books or visit museums for getting an idea what you may collect.
Isn’t it a beautiful idea, to collect dancing girls on airline posters only?! Music is in the air tonight...
Another theme could by "wild animals" (see examples above)...