Ritchey c260 seems like the best modern Alu stem I've tried, but it's pretty ugly. But if you're going to use carbon bars, it's much easier on the clamp area. Big improvement on the older WCS stems in terms of stiffness.

I run a 135mm Syntace F119 and I find it to be the best mix of weight and strength. Only the bar hooks bend with it when I'm sprinting. I find all mid-weight Syntace stuff to be really excellent. And it's a 6° stem.