It's super hard for me to choose a favorite in this area, too.
I always have a bottle of Tea Tree Oil in the house but use it sparingly as it can dry and irritate when used neat. It works wonders for fire ant stings for my daughter and so
many other things.
I also try to keep Chamomile, Lemon Balm, Skullcap and Peppermint in the house almost constantly for their various nervine effects. lol Chamomile is also a gentle diuretic so I have a few cups when I think/feel that a diuretic might be in order.
I've read some really great things about andrographis. It sounds wonderful, but for those of us with autoimmune dysfunctions like Rheumatoid Arthritis, Lupus, etc. the use of this herb could increase/worsen the symptoms as it is thought to increase the activity of the immune system.