I’m sorry I’ve been taking so long to answer this. It’s only coz I really like the question and want to answer it properly!
There are many medicines we use nowadays that have been isolated from herbs used previously to treat illnesses. The problem with using plants directly is that you do not know how much of the active compound you are taking so you may take too much and overdose or too little and it will have no effect. The amount of the active compound different parts of the plant can vary depending on where and how the plant was grown, when it was picked, how it was picked and treated after picking. Then there is the question of how to take it: cooked, raw, in tea, as a paste on skin? There is also the problem of growing enough to meet demand, which can be difficult.
One excellent example is digoxin found in foxgloves, it can very easily kill you but at the right doses it can be used to treat many heart conditions, including heart failure.
Sometimes we can isolate an active compound from a plant and improve it. Willow bark was used as a painkiller. The active compound (the one that caused the effects) is called salicylic acid which, though an affective painkiller, can cause gastric bleeding (not fun). This compound has been slightly modified in acetylsalicylic acid which is more commonly known as aspirin.
Many herbs have been used to give us out modern drugs and many more are being investigated. If I had to pick the most used, I am tempted to say the opium poppy. Its Latin name means sleep bringer. It is where we get morphine and codeine which are very powerful and commonly used painkillers.