It's impossible to discuss guitar sound as an isolated occurrence. In any band or on record, it sits together with other instruments in a mix, and that relationship determines what works or doesn't work. Any guitar sound will sound different in a mix than it does on its own, and it needs to work with the bass sound, the keyboards and the drums as well as the other guitar tracks. Sometimes the mixes work, while the guitars sound less than stellar. It's a balancing act, and it really is the complete sound that matters.
I'm not really a fan of the old school production on for example Piece of Mind. I prefer my drum sound sampled and processed, with a real punch, and I want more bass in the guitars - a palm-muted guitar should really be felt as well as heard. I admit to only having listened to the 1998 CD remasters, and it may be an issue introduced there, but the guitar sound is way too mid-range heavy for me. The mixes work well - exceptionally well even, but I don't like the guitar sound or the drum sound that much.
I must say I really only have one favourite - Brave New World. Highly processed and very modern, but miles ahead of any other record they've done. Mixes that sound very good, and I also like the sound of all the instruments. The guitars sounds so damn awesome. Janick's lead following the vocal line in the title track - it sits so perfectly in the mix that I get goosebumps listening to it. The galloping rhythm of for example Out of the Silent Planet is more scooped, but it's done in a way so that it works in the mix rather than disappear into a muddy mess. What is changed? The bass has less room and isn't heard as much as the guitars do share more of its frequencies. I prefer that way of mixing, I don't want to hear the bass 'klonk' - I want to feel it.