Can you tell me what is the best supplement for low ferritin level?

Find out why: You need to find out why you are iron deficient. Unless you're a young woman with heavy monthly losses, or are on an iron-poor diet (most vegetarians do need to supplement), you have stomach or colon cancer until proved otherwise. Red meat or ferrous gluconate / sulfate tablets will both supply iron, but first find out what's really going on. Best wishes.