If I could only have one herb, it would be Thyme. Not Tarragon, not Rosemary. Certainly not Parsley which is pretty but has too little flavor. Thyme has the richest, earthiest, deepest flavor (technically, fragrance) of them all.
It's a Mediterranean plant (Thymus vulgaris) but the Romans spread it around and it can now be found growing wild in the US. At the Farm, it grows wild in dry, nutrition-deprived meadows and makes a soft and fragrant footing as long as you avoid the bees. Wild Thyme and garden Thyme are the same things. Around here, there is Creeping Thyme (good for between flagstones) and regular Thyme. It's evergreen, and has no problem with rough winters. All Thyme wants is bad soil.
There is no need to harvest Thyme except when you need some. Even in winter, it stays fresh and fragrant under the snow. I usually use whole twigs in a recipe, not leaves. You can pull the twigs out later.
This year I am growing more herbs amongst the perennial flowers as they do in Spain. I have figgered out how to help Sage survive feet of snow.