Each game is different,. with different tech paces, leader rolls, postioning, diplo situation, etc. so most of the time there are only preferences for Lib but I would take something else if I had to (Lib race is closer) or later (blowing the AIs away in tech or tons of saved up GS for bulbs).
Generally the priority (descending order) for me is:
Military Tradition - fastest way to get Cuirs is to skip this tech with lib and use scientists to speed up acquisition of Edu + philo. Nationalism and Gunpowder can be self-teched easy enough if the AI won't trade.
Steel - an alternative for when Ais may be too prepared for Cuirs by the time you can mount an attack, and my preference for intercontinental warfare in any case since cannon viability lasts so long. Being ready for a Chemistry bulb helps a lot with making this choice over MT too.
Nationalism - for those faster tech race games where waiting too long would let a tech speedster like Mansa or Willem snag Lib, it's still good value as trade bait despite being less advanced, and you can use it to transition to drafting later or take a shot at the Taj
Communism - don't do this very often as it requires extra scientists to help bulb through SciMeth quickly, but it's very strong to pick up State Property as early as possible as I almost always play expand heavy, and it gives one a better shot at keeping Kremlin out of somebody like HC's hands who is just gonna sit on it and do nothing but deny it to you
Scientific Method - When short on scientists and trying to get to Communism quicker. Otherwise a bad choice, I find, as trading it leads the AIs to drive towards Communism or Physics themselves because of the GP rewards...airships are quite annoying.
Biology - strictly speaking its stronger than communism if you aren't very big, it's better trade bait, and it really powers up your whipping when all those CS irrigated tiles suddenly turn in 4-yields everywhere. Still requires more scientists, like Communism
Astrology - I have taken this before simply because I could not get it quickly through bulbing quicker on intercontinental maps (poor GP pool luck). The value depends heavily on it being the map type where Astro is strong though.
Constitution/Corporation - probably the worst choice as the AIs will tech these handily, but it does offer a step up on the path towards Assembly Line and the AIs tend to be quite stingy with trading Corp (since it unlocks "wonders" i.e. the Corp HQs)
Replaceable parts - I did this once going for draft rifles. Rifling itself would have been a better choice; it's not hard to trade for RP.
Don't think I've ever Libbed Democracy. There's nothing pressing about the tech itself that the advantages Communism (state property) or trading for Constitution (Representation) don't make up for IMO.