With US backing, no doubt Israel. Because US transfers technology to Israel to allow it to produce US systems that are slightly modified to Israeli preference at home. This includes Missile systems, drones, armed drones, guided munitions, infantry weaponry, ballistic missiles, etc....
Weapons such as ....:
Etc....are all composed of US tech for most part.
Israel doesn't confirm to international community that it has armed drones.But it does, I've seen them used myself in Gaza back in 2003.
However, without US major military support Israel would be weakest nation in region.
Second after Israel would be Egypt due to its experience and not capabilities on paper. Egypt has scaled down versions of US weapons which keeps it well below Israel. Third I would say Iran, since they utilize strategy well with cheap weapons. Not sure if they can wage war. They may sustain war but offensive capability isn't great.
Fourth would be KSA. Because They haven't been put to test and God forbid won't need to do. I believe KSA should increase ground forces and do more rigorous training for forces. And then improve NAVY. Then if it begans assessing its capabilities in areas in Yemen for example it could prove its influence and become regional power.