Is it worth beelining to liberalism to get the free tech (I know, I know, it depends

) I read this a number of times I would not want to ignore a few techs simply to get Lib. Also, would it be accurate to use the free tech on the most expensive one?
Usually a bee-line to liberalism is worth it, yes, but not always. Techs get more expensive as the game progresses, so getting a mid-game tech along the way for free can be very advantageous. The techs along the way are mostly worthwhile as well. They are Philosophy, Paper, and Education. (If you can generate enough Great Scientists quickly enough, you can use them to lightbulb the first two and most of the third one while you research other techs on your own.)
Philosophy is, in my books, the weakest of the three. It enables the Pacifism civic, which may be worthwhile if you're running a specialist economy; the Angkor Wat world wonder, if you have built several temples and would benefit from running priest specialists; and you found Taoism.
Paper enables the trading of world maps. This could allow you to win the circumnavigation race without having to do all the exploring yourself. In addition, selling your world map for gold a few times over can bring in a good amount of cash. You can also build the University of Sankore--handy if you've built several religious buildings. It also opens up access to Printing Press.
Education enables universities and Oxford, buildings which will boost your research. Besides opening up access to Liberalism, it also opens up Economics (free Great Merchant) and Gunpowder (though Guilds does that as well).
Finally, once you have Scientific Method, Liberalism is a pre-requisite for Communism, which enables the State Property civic (invaluable if you have a large empire), the Kremlin world wonder, intelligence agencies, and the first one to research it gets a free Great Spy.
As henrebotha mentioned, all the Liberalism-path techs are excellent for trading, especially because the AI doesn't tend to prioritize them as much as other techs. So you can use them to back-fill techs you've missed. Just don't go trading them away too soon after you get them, otherwise the liberalism race will suddenly look more accessible and attractive to the AI. I usually don't trade away any liberalism techs until I have the next one on the path.
As for which tech to choose for free--choose whichever one will provide you with the most benefit in your situation. Astronomy, for example, is probably the most common Liberalism tech choice because of its expense. However, if you're playing on a Pangaea map, Astronomy's benefits are considerably reduced and you may want to select something different. Nationalism is another popular choice as it allows you to get a head start on the Taj Mahal. If you have a significant lead in the Liberalism race, you may even want to research Liberalism to within one turn of finishing then research other techs to open up something even more advanced like Chemistry.