If effectiveness against attackers is what you are looking for, then I'd suggest to look outside of Aikido for that...it offers a slower learning curve than other approaches if self-defense is your priority.
If you are looking for effectiveness within Aikido, I agree with the post that it is all about the instructor.
That said, there are some styles which generally lend themselves to developing effective technique in a shorter period of time than the Kokikai or typical Aikikai approach.
You might check out Yoshinkan, Yoseikan, or Tomiki Aikido if there are any representative dojos in your area.
I mis spoke by saying effectiveness against attackers. I was trying to get others opinions on which style they thought was "better" and which style offered a more traditional Aikido. Without a starting a "my style is better than yours" debate. LOL Anyway I appreciate all of your advice thank you to all.