Given you've just reached everything you need for Cavalery, there are several options:
Communism
Everything regarding that tech that has to be said was mentioned already. If you've conquered great amounts of land, it's one of the best beelines, especially if you managed to not trade Liberalism away and have a shot at the free great spy.
Physics
Usually not my tech of choice, but it offers one option, namely airships. IF you already conquered great amounts of land and want to beat down another small empire with your Cavalery that has Rifles, this might work. Needless to say that you'll need great numbers and your losses will still be high. If you're not aiming for airships, i'd rather try to trade for the tech (unless you're aiming for artillery).
Biology
Usually my favourite beeline tech once i've conquered alot of land - with that much food, you can make almost EVERY city productive, and with Representation, your techrate is great even without any cottages. Also the "mid/lategame Aesthetics" regarding tradevalue, as the AI often does not pursue it. If you have Biology, even whipping Infantry and Artillery becomes reasonable - not to mention if you already have Factories+Plants and maybe even Kremlin, which makes whipping ridiculously efficient (up to the point where you can whip massive armies consisting of tanks). Clearly one of the best beelines, unless you're playing mainly waterbased maps - but even then, it opens up Medicine - which, with Sid's Sushi, is maybe the best choice for Archipelago or alike.
Assembly Line
Unless something odd happens, it's usually the #1 tech to beeline for me. The production bonus is just too strong to pass by, and Infantry often are a superior military unit. There is almost nothing to do wrong with that tech, maybe if you're aiming for Diplo you're better off straight beelining Mass Media, but that's about it - every other VC will most likely demand AL sooner or later, especially the military VCs where you need a large navy to conquer other continents. Destroyers and Transports are just too expensive otherwise.
Astronomy
As it's not on the Cuir/Cav beeline, i thought i'd mention it. Pretty obvious tech, but i'd be careful with it: given you've conquered your continent, opening borders with small oversea empires will give your enemy great amounts of traderoute commerce, while you won't profitate that greatly whatsoever. Other way round, for example when you're pursuing Diplo and are small yourself, it's definitely a great tech to go for.
Steel
Pretty obvious: either you need cannons for a military VC, or you need Ironworks for Space. Also a good tradechip with high beaker value, although you rather wouldn't want AIs to have cannons - even when they're terrible on massing siege.
Combustion
You need a navy? You need combustion. One of the main beeline techs on waterbased maps, and if you have it soon enough it will net you the victory there. There's nothing (i repeat: nothing!) an enemy civ can do against oil based ships with everything that sails. You can starve them to death by blockading their cities, pillage them back to the stoneage, make quick surprise attacks on poorly defended cities and so on. Ofc also a very valuable beeline as Mining Inc. is on the way, a great choice for waterbased maps overall.
On more landbased maps, it's usually not too much of importance, although it opens up tanks and bombers. If that's what you're aiming for, well, go figure.. this combo is surprisingly effective also vs. bigger and technological even enemies, as the AI can handle 2-movement-units with air support very badly.
Mass Media
When you've conquered your continent, and the other continent loves you, why bother to conquer all of it? Just vote yourself to victory. Other than that, this beeline has little value, unless you're aiming for a lategame culture win (which actually is surprisingly effective, but is endangered by large, sprawling empires that have a lust for blood - and in the lategame, the technological disadvantage will kick in even more. Not to forget some AIs love to beeline those techs, and you'll need at least some of the wonders in order to make up for some of the culture you'll lose from researching Sci. Methods).
Democracy
Thought i mention that one too - NOT worth beelining, unless you're already very far ahead and really want that freakin' Statue of Liberty for some odd reason. Maybe you're even so far ahead that you HAVE to research it in order to progress in the tech tree, dunno ... other than that, just ignore it and trade for it later.
Flight
Controlling the air will give you an decent advantage for a loooong time, BUT the AI loves to beeline it aswell, so make sure you go for Bombers (Radio) soon enough to fully leverage your advantage. With Bombers, you can roflstomp over Rifles and even Infantry with your Cavalery - if played out right, they're tied to #1 siege unit with cannons from what i've seen, and they're a save bet up to Immortal difficulty.
Rocketry+Fission
Now you've conquered your whole continent, but the enemy seems to annoy you with keeping up in tech and production? Just sweep them away with the famous nukes! All you need is some production to set up some nukes fast enough, and some units to conquer nuked cities. If you happen to have a monopoly on those techs, it's the easiest and most cost efficient way to win Domination/Conquest. Also a good choice if you're desperate and want to turn a otherwise lost game around by any means. Submarines are kinda essential when you want to conquer another continent here.
Late-late-lategame:
- Plastics->Computers: You're behind and still want to go for space? The Internet!
- Genetics: You're far ahead and just want to play it to an end? Genetics for the
- Computers->Robotics: Mech. Infantry are extremely powerful and rather easy to beeline, and they come soon after the Internet, so they might be a good choice. Still, they'll need siege, so either you have bombers or you'll need mobile artillery, which is another huge detour.
- Computers + Composites: Well, there's not too much to say about the single strongest land unit in the game, Modern Armor ... they'll mop up your enemies with raw firepower. But usually your enemies will have some really tough defenders, so unless you mix them with Mobile Artillery, it's nothing worth beelining for.
-Stealth: Pretty much the same problem as with Modern Armor - Stealth units are very strong, but they come too late to simply beeline them for an military advantage. But otoh they're fun to use and make modern era warfare a breeze if you happen to have them soon enough.