My name is Ryan Goyette, and I am purple belt Brazillian Jiu Jitsu competitor. I have been training for 7 years and competing for two years. I’ve also been lucky enough to stay away from any long term injuries during my time in the sport.
It’s hard to say what specific techniques are the best for multiple reasons. For one, everybody’s leverages are drastically different, what works for me could very well be useless to somebody else. Along with leverages, people’s individual gameplans and sequences are going to vary to a great degree. Purely from the competitive standpoint, I would say that having a strong wrestling or guard pulling system, having a great Torreando passing sequence for an open guard opponent, and a great sequence of positional escapes/sweeps on bottom will make you a deadly competitor.
Though there is a large variance in strategies one can use to win competition, Self Defense requires a different approach. You need to consider weapons, striking, pavement slams, legal ramifications, and the number of opponents. If your opponent is drastically larger than you are, they have a weapon, or they have buddies, you need to become a phenomenal runner, and have 911 at the ready. If there is no avoiding an altercation with some drunk nutjob, your best bet is to obtain some sort of body lock clinch, take your opponent down, and mount them until help arrives. This keeps you from being hit, it fully controls your opponent, and it saves you a lot of legal heart ache.
That said, your best technique to dealing with any competitive or self defense scenario is consistent training. If you no confidence executing your sequences on resisting opponents, your hardwork and research is useless. And when it comes to learning techniques, the best individuals to reference for BJJ competition are John Danaher, Gordon Ryan, and Mikey Musumeci. For self defense, John Danaher, Bas Rutten, and Khabib Nurmagomedov. In general, you should always study the best of their craft, or individuals who have developed the best of the best from the ground up.