Idk, I have tried every ISP and have an NS2. NS2 works just fine as long as your amp doesnt mind the 4cm.
I personally feel the $200+ pedals would be splitting hairs and wouldn't be that much better. But I am sure the Zuul and cockblocker will be considered "the best"
I cant comment on the NS-2 but I've owned the ISP Decimator G-string for years and it has done a great job for me. I've used it in the in the "4 cable method" by putting it infront and the loop of the amp (Splawn Nitro) and has quieted the amp down nicely without affecting sustain or tone.
I use the Zuul, but also like the ISP and can't really tell a difference to be honest. But I also play almost 100% through a Suhr Reactive Load and my studio monitors. So I don't really have to deal with a loud 4x12. But either is going to get the job done I believe. Can't speak for many other gates though.
ISP is my winner so far. I don't use them anymore, as I have put NOS Phillips/JAN, GE, etc. 5751 (quiet variety of 12AX7) in some amps and it has quieted them a bit. At least enough for what I do.