I'm a fan of the Kalloy UNO line of stems. Well made and super light. A couple of years ago I was involved in a somewhat silly challenge: build a 16 pound Road bike for $1,500. The UNO stem was my weapon of choice here.

I researched carbon, and they were mostly heavier. Many of the so-called carbon stems were just alloy with a cosmetic carbon wrap.

Many of the online-sourced high-level stems were and continue to be fake knock-offs. Some premium brands are massively pirated, even at a retail level.

Only downside of the UNO stem: it is four bolt. Two bolts on the faceplate are superior. The larger bolts are stronger and harder to strip. More importantly, the two bolt stems make it impossible to have tension differentials between the bolts. With 4 bolts, you have to carefully torque each bolt individually.