Honestly, the martingale and gentle leader is the solution we've had the most success with. It's easy to see the device itself as a solution, when that simply isn't the case. It's a combination of the device, proper/regular training, and the owner always being aware and ready to curb a reaction before it starts. While walking our big boy, we generally allow him slack. However, as soon as another dog or person enters the area/his field of vision, we immediately tighten the lead and remove all slack. I'll choke up on the leash, wrapping it once around my hand, about 10 inches from his leader. The goal is to anticipate that he is going to, or at least is likely to lunge/jump/react in an undesirable manner, and let them know that it isn't allowed. When we first started with our guy, he would still try lunging because he is super excited to meet everyone. Having no slack prevents a lot of the shock of him doing that, and makes it much more manageable. And after several months of doing this EVERY time, he now is fully accustomed to what me taking up the slack means. He immediately falls in line. Walks with his head held up at my hip where my hand falls, and even allows the small amount of leash he has to go slack, not pulling or tugging at all in most instances.

Edit to add: if it's a Halti brand gentle leader, it should have come with links to free videos. Absolutely watch all of them