Yarrow, Plantain, and Solidago are all plants that help with various health issues. I'm pretty big on foraging, but these plants are amazing to keep around to either add to teas or make for teas.
If you are looking for something a bit stronger, Mr. T. He's less about herbal. More about kicking ass.
Mint and ginger are essential, I try to keep some sort of valerian around too.
I love tulsi / holy basil, but there seems to only be a couple of brands that do it here (UK) and they're usually a bit pricey.
Something that can be grown somewhat locally. For me it would be hibiscus flowers.
I have a hard time picking one, but Senna has some practical medicinal uses so it should probably be in there.
Decaf green tea, steeped at room-temperature, so NO tannins get into my tea...
possilby rooibos, but haven't had it in awhile, so don't know if that's still valid...
possibly something with cardamom seeds in it?
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I've been alternating between green & lemongrass+ginseng for a couple of months.
I always stock Lapsang Souchong, Earl Grey, Russian, Sencha, Bois Chéri Trois Pavillons,Yerba Mate, Bissap, Verbena and Melissa from my garden.
Chamomile(relaxation), lavender(relaxation), peppermint(nausea deterrent), and lemon(sore throats and the like). Catnip's also kinda neat when tea'd; got an interesting flavor to it, but I'm not sure if there's any therapeutic benefits to that.