This is both a tough question and an incredibly easy one. As far as deciding which systems are best, I would say those that contain movements that rely on easily learned gross motor skills--stuff that you can learn reasonably well in a short period of time and be able to apply effectively if you are attacked sometime soon. They should also be tactics that allow you to immediately understand the function they provide. If you're spending your time practicing the "Flying Lotus" or "Gargling Badger" or some other movement that doesn't immediately conjure up vivid visions of pounding the snot out of someone, you're not on the most direct path to learning how to defend yourself on the street.
In evaluating systems, you must also consider your physical attributes, limitations, and build. Sumo is a lousy art for a guy that's 6' 2'' and weighs 160 pounds. Tae Kwon Do doesn't work well for someone that's 5' 2'' and 250. That doesn't mean that ALL of the techniques of those arts are inappropriate for those people, but enough of them are inconsistent with their physical statures to make them poor choices.
Are there generic arts that offer a good body of technique for MOST people? Yes. That brings us to the second part of the problem: Finding an instructor that's willing to cater to YOUR needs.
The problem with any institutionalized matial art or self-defense system is that it survives by making people fight according to that system. It creates a progressive structure that you are supposed to navigate through to develop proficiency in the system. Ideally, those goals are consistent with your goals and you learn stuff that's relevant to keeping yourself safe. If those goals are geared more toward fitness, competition, cross-cultural experiences, or other directions that do not support learning how to hurt your attacker and get away, you're not taking the shortest route to developing the skills you need.
For example, Brazilian Jujitsu is an awesome art that has much to recommend it. As a competitive art and a basis for developing combative skill, it has a lot to offer. However, if your biggest fear is going to the ground in a streetfight on concrete, learning any tactics that emphasize grappling on all fours (knees and elbows on the ground) may be very counterproductive. When you fight the way you train, you stand the chance of destroying your elbows and knees on the concrete.
Please understand that this is an example used to illustrate a point--not an attack on BJJ. It was based on actual experiences with one of my private students during a training session that "went off the mat."
So how do you get what you want? First, define a clear set of goals for your personal defense needs. Then start visiting different schools and discuss your goals with the instructor. Tell him that you respect his art and his approach, but you want to focus on your specific goals. If you see his students doing something that you feel is impractical (i.e. jumping, spinning kicks), tell him that's not for you and see what he says. If he's not willing to work with you and your goals or tries to lead you off into the martial arts weeds, walk away.
For practical self-defense, some arts are definitely better than others. Unfortunately, as we say in Martial Blade Concepts, "If you try hard enough, you can f%$& up anything." You may find a great kung fu instructor that meets your needs better than a Krav Maga instructor that isn't representing the full potential of his system.
Remember, it's your butt, so you get to choose.
Stay safe,
Mike