At level 91 thieving, you unlock the 8th floor of pyramid plunder. If you only loot the 7th and 8th floor, so the 2 highest ones, you can get up to 270k thieving xp per hour which is the fastest in the game.
This method is the recommended way to train from 91, and youll profit a bit getting to 99 as long as you get a pharohs sceptre.
There are quite a few thieving methods that come close to that xp rate though, so you can switch it up to make your training more interesting, but pyramid plunder is the fastest.