Chamomile — the reference point
Forgiving, familiar, and the flower everything else is measured against. It teaches one thing immediately: whole heads and milled dust are the same plant and not the same drink. Steep off the boil; boiling water and ten minutes is where its reputation for bitterness comes from.
Rooibos — a drink that behaves like tea
Takes milk, tolerates a long steep, and has no astringency to manage. It is the answer to 'something like tea without caffeine' and it teaches that processing rather than plant is what makes a drink tea-shaped: rooibos is oxidised in heaps, which is why it is red.
Ginger — the first thing you simmer
Ten minutes at a simmer rather than three in a cup, and the difference is not subtle. One botanical taught properly settles the whole root category, which is the single most useful rule in this catalogue.
Peppermint against spearmint — one purchase, two plants
Buy both and taste them together. It is the most useful discrimination in the herbal aisle, it takes ten minutes, and afterwards a packet labelled simply 'mint' stops being a mystery.
Hibiscus — sweetened, as everyone drinks it
Sharp, red, and the base of most red fruit infusions on sale whether they say so or not. Drink it sweetened, because every tradition that drinks it does, and judging it unsweetened is judging something nobody makes.
What five teaches that fifty does not
Plant part, preparation, quantity, format and the gap between smell and taste. Those five ideas cover the rest of the catalogue, and there is no sixth idea waiting behind the next twenty plants.