I use a collection of numerous sites. I have them all bookmarked under an AC folder in Firefox. I find some sites have better guides then others, so I just use which ones I prefer.
Thinly is the main website I use for guides, the acnl wiki to see what bugs are available on what day, and tumble for new room ideas and for qr codes ~
Tumblr, specifically bidoofcrossing. Found out about it in Chuggaa's LP and have used it religiously ever since.
I use Thonky for villager birthdays and bug/fish stuffs.
Nookpedia for a villager list with pictures
ACNL wiki for other general information
The GameFAQs community as my black market
And this forum for guides and generally everything else~
besides this forum, i also use thonky, certain guides from jvgs, and bidoofcrossing on tumblr (which sometimes leads you to a bunch of source sites for qr codes!). i'll also just google random stuff and click on the first couple sites that come up otherwise.
Besides TBT, I like using Bidoofcrossing, Thonky, Nookipedia, MoriDB (not just for wishlisting, but it also helped me with the theme challenges once I found out that Thonky's list wasn't accurate).
I like to use this guide to see what a villager's house looks like before I have them move in. That way I can decide if they'll fit in with my town theme, and what kind of decorating I should do around their house for things to look the best.
That kind of thing's not for everybody, but it's a guide that I personally like a lot ^^