What is the best item you can get while thieving on Runescape?
I would've listed a few, but since you asked for the best item, it would be the Pharaoh's sceptre. The current value for it on the Grand Exchange is 820K, and it is a rare loot from the Grand Old Chests in Pyramid Plunder thieving.
The best item you can get from thieving is: 100/400 coins, noted bronze bar, noted iron bar, noted steel bar, noted mithril bar, noted adamantite bar, noted runite bar, noted tin ore, noted copper ore, noted iron ore, noted mithril ore, noted adamantite ore, noted runite ore, noted coal, level 3 clue scroll - Which is from a Dwarf Trader in Keldagrim which you need level 90 to get. Hope this helps :) Good luck thieving.
From pyramid plunder, pharaohs sceptre/scepter. Around 1m I believe.