Boxing is proven to have the best punching out of every martial art and it's fair to say that the average boxer could beat the average karate practitioner. Not only because of the art of boxing itself, but the way that we train. Also, boxing is very simple; there are only five ways to throw a punch in boxing and you train to specialize in that alone. Anyone can learn how to box because of this and it doesn't require the flexibility of throwing kicks like many of these other martial arts (something many people lack). I agree with your point that it ultimately depends on the fighter, but it would be foolish to disregard the fighting style itself. I'm sure you're well aware that some fighting styles were designed with the intent of countering another type of fighting style. Correct me if I'm wrong, but Wing Chun would fall under that category as would Brazilian Jujitsu.
Speaking of which, jujitsu would, again, be practical to learn because it's one of the most effective (if not the most effective) martial arts that utilize grappling especially when you're taken to the ground. I'd even go as far as to say that the average jujitsu practitioner could be the average striker in most martial arts (including boxing). If you don't believe that, you could watch some film on Royce Gracie as he pretty much proved how effective it could be against people regardless of how big they are. Hell, I can personally attest to the effectiveness jujitsu in a street fight (but keep in mind that I'm also 6'1" 170lbs).
You also have to consider who you'd be fighting against. The average punk on the street doesn't know how to actually fight. They mostly throw wild haymakers that any decent fighter could see coming. Also, they almost never throw kicks. For that reason, boxing would be very practical to know. Of course, the chances of a street fight going to the ground is good, which is why it would be a good idea to know what to do should that happen. If it's one on one and your aggressor doesn't have anyone that'll jump in, use jujitsu to pacify him or you could just land a couple decent punches and hopefully he'll back off. If it's more than one aggressor, (you should run) boxing would serve you better than grappling.