Thomson stems are great, but only go to 130mm. The old 3T ARX stems with 2 bolt steerer and 4 bolt faceplate were right up there just under Thomson for stiffness and can be had in 140 & 150. They're rare these days but they do come up 2nd hand from time to time. I have them from 120 up to 150mm and will never part with mine!
Any well built alloy stem is going to be more than stiff enough for 99.9% of riders. Using a well built stem, any flex you feel will most likely be in your bars or forks/steerer tube or wheel.