I used both kinds of Shin Etsu (G751 and X23) and they're absolute garbage. I was seeing load temps 10C higher than Arctic Silver. They were also very expensive when I tried them (2013 timeframe), like $10 for 2g of G751 that came in a tiny ass applicator and was enough for maybe 2 applications. This was with a Thermaltake Frio (OG version) air cooler. The temps were absolutely awful. I'll never use that brand again.
I use, and swear by, Prolimatech PK-3 Nano now. (It is "nano aluminum" whatever that means). I saw a small temp improvement vs AS5 but more importantly, it is non-conductive
, cheap and easy to work with and clean if you are doing multiple remounts or something. I got a huge 30g tube of it like 3 years ago for $30 and probably still haven't used 20% of the tube. THAT'S what I'm talking about, that's my money's worth.
I have heard that MX-4 is very similar to PK-3 Nano but I haven't tried it. I've seen videos and it does look similar in color and consistency. So it's a nice alternative. It is around the same price last I checked too.
I have used IC-7 Nano Diamond and used to recommend it except that it is the worst stuff to try and spread, it's too thick, it's difficult to clean off
and it will literally discolor and damage your IHS (integrated heat spreader...)
(emphasis added). It literally stained one of my Phenom II's over time. I even heard that it's basically corrosive and can eat right through your IHS and drip onto the die and damage it, ruining your CPU. I have no idea if this is true but that's what someone around here claimed. There was some kind of expose on it with the president of the company or something too, admitting that it damages CPUs over time. Regardless, the temps I had with it were the best compared to AS5, PK-3 and the other pastes I've tried.
Liquid Ultra is the liquid metal type stuff right? It's expensive and you only get one application. If you're bad at mounting heatsinks or waterblocks like I am, or just a perfectionist and remount coolers to get the best temps, or you're a bencher with a dedicated bench and change CPUs all the time, then this stuff is worthless. Many reasons not to use it.
TL;DR: Just get a huge tube of PK-3 or MX-4 and be set for years. The temp difference over AS5 is only like 3C, and probably also like 3C within the Shin Etsu, except the PK-3 and MX-4 are much easier to work with.