There's a couple of guides I still use and find pretty useful.
I think the one I use most is
Ultimate Text Color Guide it's so useful for finding colours for RPs, sigs, etc.
I also like to use the:
Master Chance item list when I forget the name of an item but know it's from a RIG.
Free item guide for free item links I tend to miss otherwise.
Achievements guide, i use this one to find out/double check how to get certain achievements.
Master Guide of Guides is good to see what's out there in terms of guides, and is far easier to look through than using the Gaia search function.
There's a couple others I use that I can't think of right now, I've never made one myself though it seems like a lot of effort (which makes me appreciate it even more when other people make guides)