There's no blanket "this works best" for all engines statement that can be made. Be cautious of those selling snakeoils with this claim.
But there are some general rules of thumb for performance that you can follow. The most basic being that an engine is an air pump, and anything that restricts its ability to pump air hurts performance.
Most cars restrict performance on the exhaust in pursuit of quiet and cost. Small diameter pipes and cheap mufflers usually restrict flow. Poorly designed exhaust manifolds can really hurt engine performance and efficiency.
On the intake side, many older cars have very restrictive inlet snorkels, and draw hot low pressure air from the engine bay. Cooler denser air drawn freely into an engine helps performance.
Camshafts define your engines personality. You balance peak numbers with usability. The more radical the lump stick, the harder it is to live with, but you get a higher peak number...usually...if you set the rest of the engine up to use the radical camshaft.
Porting and polishing are all about flow. Most stock engines are adequate on this department. A good valve job including a 3 angle or more cutting does gain you some flow. But all by itself, you wouldn't feel it in the seat of your pants. Narrowing down valve stems and such doesn't really help an essentially stock or even well warmed over engine. And it's expensive. This sort of work really shouldn't be done until the rest of the engine calls for it.
Compression increases are usually good, but beware of accidently restricting the flow through the combustion chamber! Many times you will do better to use a higher dome piston than to shave the head.
The next generic statement would be about fuel mixture, get it right. Many a carburetor upgrade merely has to do with getting the fuel mixture more correct, and nothing to do with enhanced flow. The SU doesn't flow any better than the ZS for example, but it's easier to get the fuel mixture right for performance with the SU for most folk.
And lastly is the tuning of the ignition timing. From where it starts, to where it ends, and how it gets from one to the other. Many an LBC has a good bit of power to be found here, particularly the later smogged ones. Merely advancing the base timing can gain appreciable performance increases, for free.