Is Krav Maga more effective than Brazilian Jiu Jitsu? ?

You are comparing apples to donuts.
Krav was designed to military and defensive applications for extreme situations in areas where terror attacks are common occurrence. And whose populace are conscripted to a minimum term in defense of their nation. So all citizens who have served have training is such a martial science which is extremely effective and have the actual combat time to prove.
BJJ is NOTHING BUT A COMBAT OR MARTIAL SPORT IT IS NOT A MARTIAL ART, BUT A SPORT.

FYI trying to compare something that is designed for the battle field and compared to something that is created for entertainment purpose and value is futile and foolish.

What makes any martial art effective is not the name, but the person who teaches it.

depends on what context. In a combat match with rules, bjj wins every time. In the street, if you have trained in real krav maga, not the watered down version, you have a chance to win, but bjj can have a chance too if good 

In the ring, no. In the streets, yes.
The core problem with BJJ for self-defense is that the ground is the last place you went to be in a street fight. If your opponent has a buddy, your head is getting kicked. If your opponent is concealing a knife, you're getting stabbed. When you're in a real fight, and not in a controlled MMA match, you want to be on your feet where you can either defend yourself or be able to escape, you don't want to be rolling around on the ground where you're a sitting duck.

That depends on what you intend to use it for. Krav maga contains elements of jiu-jitsu, but unlike jiu-jitsu, it was invented by Israeli special forces as a martial art to be used offensively, as well as defensively. Jiu-jitsu, on the other hand, was designed to be used defensively, not offensively, offense being contrary to the ways of jiu-jitsu. 

So, if you are a pacifist and you have dedicated yourself to strictly using your martial art to defend yourself, to take down an opponent who is attacking you, then jiu-jitsu is likely to be more effective, but if you aren't a pacifist and want to be able to go out and attack someone, to be the attacker, and take them down, then krav maga will be more effective. Like I said, it depends on what you intend to use it for.

No, I haven't watch winners in UFC who practiced Krav Maga.