You cannot simply label one Cult Terminator as the "most effective" without saying what the effect is that you are trying to achieve.
Noise Marines can lay down a lot of firepower, but I find it frustrating that they have such low (or rather, high) AP values. I play a lot of Tau, Necrons and power armored enemies, so I don't see their value. If I played against a lot of IG, 'nids and Eldar / DE I would maybe have a different view.
Plague Terminators are tough as nails. Their T 5 combined with the 2+/5 (I)+ save makes them close to indestructable. An Aspiring Champion in that squad with a Plague Sword would be especially ugly.
Along the same lines of tough to kill, you have the Thousand Son Terminators. Only weapons S5 and above can hurt them, and they have two wounds (which makes up for the loss of the S5 rule once they get into close combat), with their Terminator armor save on top of that. Their "Slow and Purposeful" is redundant with the armor in place, and it is frustrating to not be able to upgrade their squad weapons. On the other hand, a tricked out Sorceror in Termie armor leading the squad can be very powerful (Flames of Tzeentch a-go-go!).
Berzerker Terminators are hard hitting, but don't have the staying power of Thousand Son or Plague Terminators. Their S 5 attacks means that, statistically speaking, 8 Berzerker Terminators against 10 Space Marines would wipe out about 6 or 7 Space Marines without losing one man. Compare that to the results of 7 Plague Termies, which is about 5 dead. The bonus is also felt against lightly armored vehicles or rear armor (goodbye dreadnought!). Let's not even go into an Axe of Khorne on a Termie Aspiring Champion...
In the end, here are my views:
If you want to cut light infantry to shreds, go with Noise Marines.
If you want a unit that can march across the field, take a severe punishment and still deal out death, go with Plague Terminators.
If you want a unit to stand and fire, take Thousand Son Termies.
If you want a unit that will hit hard but short, go with Berzerker Terminators.
I could give you some interesting comparisons of Plague Terminators vs. Berzerker Terminators...but my Blood Brothers may be angered.
Let's hear what others have to say, and then I'll give some more input.