Mainstream coins or altcoins. I mean it's bitcoin and then everything else is altcoins so I think you need to figure out what you are asking first. But if you are talking large cap vs small cap coins it all depends on your goal and appetite for risk. There is not one size fits all answer here.
It's up to your risk tolerance and how much money you have to invest. I'm poor so I go for newly launched projects only... Shardeum, Lamina1, Layer0, Q Blockchain, Zeta, QUAI, Mantle, Monad, Scroll, Taiko etc. Lower cap means higher potential returns. I need a moonshot!
I don't understand why people keep saying that a low-cap token has a higher potential for growth when it basically doesn't have anything to do with the growth or potential of the project behind that token or coin. How can you be so sure that a token will perform well just because it has a low cap which is basically because the price of the token is low or the supply is extremely large both of which don't really prove that it will grow very high in the future?
It's a different thing if you first evaluate the project and then check and see it has a low market capital and you think that it might go up because the project is good, but just because a token has a low market cap it doesn't mean it is going to get significant success in the future and a lot of people think that way.