I would also go for the train. There is nothing cheap in Switzerland and that includes taxi services. And if you want reliable on top of that, it will cost you still more.
The trains are way cheaper than any taxi service, almost as fast (the IC trains between Lausanne and Geneva Airport only stop once in Geneva station) and reliable. Geneva Airport station is, as its name suggests, in the airport.
Buy your tickets online at
www.sbb.ch, a kiosk at the station (the kiosks work in English), or at the ticket office. The website will give you schedules too.