I am still a relatively new at stamp collecting as I have been doing this for only 3 years. For the US stamps here are the issues that have had a healthy premium:
A) Priority Mail and Priority Mail panes. They have appreciated quite well and these used in very good shape are being discovered as scarcities for some years.
B) Used low denomination stamps ( 1 - 10 cents) are become less and less common than in years past. Part of the reason is the fast rise in the price of first class letter stamps --- in 1972 it cost 8 cents to mail a letter so you could easily use low denomination stamps. At the rate 66 cents in 2023, it would be hard to find space for 66 one cent stamps! So a potential area of collecting which is rewarding for both the designs and formats available as well as potential premiums on them (albeit modest).
C) Commemoratives can develop a premium but the theme has various sized niches -- anything to do with astronomy and space exploration does do well although over time the premiums have gone down. With current stamp issues' production ranging from 18 - 34 million, this may not be the case. Furthermore, used commemorative in very good shape are sometimes uncommon.
For World stamps these are the issues that have not done well over time because of overproduction and overuse of theme:
A) Princess Diana
B) Some British monarch issues (and not only from the UK, I can purchase a King George the 6th 3 cent Canadian stamp online for 10 - 20 cents).
C) Marilyn Monroe, Elvis and a few other American entertainers are overproduced though there may be a niche for them but realize these are saturated.
D) Animals --- one example is there are tons of bird issues and little premium unless there is something special about them such as the Dutch Caribbean bird stamps from I believe 2020 which were produced in very low numbers, based on uncommon, historic drawings, and quite beautiful.
E) Any country that churns out far more stamps than the populace will ever need. There are countries which see stamp production to collectors as a source of revenue for government coffers. Try to avoid that --- as well as overly gimmicky stamps (Austria is guilty of that although there many fine stamps otherwise issued by that county).
F) Commemoration of obscure government agencies or evidently politically motivate produced stamps, as well as very famous leaders (Ghandi, John F Kennedy, Simone Bolivar, Charles de Gaulle, Napoleon, Eisenhower, Churchill, etc).
Neutral cases that depend on several factors:
A) In the US we have many Flag issues that are considered definitives. Collecting an individual one MNH or used will not add much value to your collection. However, if you get a Flag stamp of a design you like in all formats and as new plate numbers are produced, you will have something of value that would attract collectors. The best way to do this is simply choose one particular Flag stamp and stick to it for awhile. Trying to do this with all Flag issues can be expensive and daunting UNLESS you are passionate about this theme. IN that case you could focus solely on US Flag stamps as a collector. It is an extremely rich area and historically interesting.
B) US Coil and booklet stamps MAY have a high premium. In the US the Washington Franklin stamps from the early 20th century can be worth much if a particular coil or booklet stamps even though a few were produced in large quantities. Why? I think it was the collecting habits then. You figure you could easily get a block of 4 from a sheet and booklet and coil formats were not as heavily pursued as they began to be decades later with the coil formats really taking off with the Transportation series starting about 1980 in the US. And to this day, coil and booklet stamps are an area pursued more by intermediate and advanced collectors as there are nuances not always found on sheets and panes.
I hope this info helps and I ask any of the seasoned collectors to correct anything I have posted.
PS. I do not touch upon errors although for modern postage that is where some extremely valuable stamps are found. Why? It is well beyond the scope of this thread and my knowledge. Unless you encounter a very obvious error (such as the perforation is definitely in the wrong place or the color of the stamp is evidently off) I would wait a few years before even buying error stamps. To give one example of how you need to be well-informed about error stamps take the Dag Hammerskjold commemorative stamp from the US issued in the 1960's. An invert error was produced which can be seen by a white strip on part of the stamp. The USPS decided to print millions of that stamp to avoid having a scarcity/rarity in the market and people hoarding the stamp. If you did not know this and some unscrupulous seller convinced you to spend $100 on the invert error you would be very disappointed to find you could get that stamp for 30 cents many other places - the same price as the original, error-free one.
(there is an entry on Wikipedia about the history of this error -
https://en.wikipedia.org/wiki/Dag_H...B6ld_invert)