Flax:
Sell: 94 gp +
Buy: 75 gp
Amount Made: 19gp per
200,000,000 / 75
2,666,666 = Amount of Flax Brought
Buying Price: 200,000,000
Selling Price: 250,666,604
Profit: 50,666,604
Nature Rune:
Sell: 212 gp +
Buy: 190 gp
Amount Made: 22gp per
200,000,000 / 190
1,052,631 = Amount of Nature Runes Brought
Buying Price: 200,000,000
Selling Price: 223,157,772
Profit: 23,157,772
Dragon Plate skirt:
Sell: 189,000 +
Buy: 166,600
Amount Made: 22.4k per
200,000,000 / 166,600
1,200 = Amount of Plate Skirts Brought
Buying Price: 200,000,000
Selling Price: 226,800,000
Profit: 26,800,000
Conclusion out of these,
i would say Flax is the best choice

Feel free to say if i have made a mistake.