I've been reading this thread, and finally decided to throw my two (uneducated) cents into that hat.
As far as MA's go, the representation of the art is only as good as the presenter. How effective it is, or isn't, also depends on the "fighters." I'm a lowly 7th Kyu, and to some of you, I will sound rather green, and I will admit that I don't know a lot compared to others on this, but some things I have been taught and learned so far, I think are missing from this post.
#1 How effective do you want to be?
In Aikido, as I'm sure with most (if not all) other arts you can always end your altercation with one strike, or hold. So which one is best? Well, do you want to permanently injure someone? Do you want to kill them? Do you just want them in submission? The answer to each of these questions probably could represent a different art for which one is "the best."
#2 I've heard people say, Aikido not effective for this situation, or that situation. I personally look at Aikido as being effective for more situations than a lot of other arts. There are thousands of moves in Aikido, and I have to make a reference to Isaac Newton with the following, "for every action there is an equal and opposite reaction." With the many techniques available, I would venture to say there is an appropriate and effective move or defense for nearly any non-projectile attack. Whether they work or not takes me right back to Newton. Even with all the possibilities, every Aikido move can be countered.
#3 It's all attitude. Why does Aikido not fare so well in "competition" situations? Because it goes against the very nature of why it was founded. I am fairly confident that you would never find a very high ranking Dan in the Ultimate Fighting Championships, or similar competition. The men, who enter and win these tournaments, train to do that. Aikidoka do not.
#4 Most importantly - speed.
In these tournaments it's all about speed. Whoever has the fastest reactions and reflexes wins. That's all there is to it. If you can land a punch, or strike or other technique on me, before I am able to react, it's game over. I don't care what art you practice, if someone is faster than you and has knowledge of the weaknesses of the human body, be ready to get hurt - or worse.
Before anyone else continues on with what's the best, or most effective MA, I'd like to see the "best" vs. the "best." But that's something we'll never see
Well, that's my 2 cents, or dollars considering the length.