Kill Six Billion Demons is my favorite right now. Gorgeous, sprawling, complex artwork from start to finish, philosophically interesting, bordering on pretentious but I love it. I sometimes read this one while wearing my VR headset to properly examine certain pages close up.
Johnny Wander is the homepage for an incredible artist/writer duo. I've really been enjoying their latest story Barbarous, but their archives are a bit of a pain to navigate to find the starting point for each story. Worth the pain though.
Octopus Pie has been over for a few years but is always worth returning to. Woman lives in New York, people are weird, and New York has a lot of people. Reading this one will make you feel good, Do Not Skip.
Sleepless Domain is an interesting one, generally a cutesy story with an anime, leaning towards chibi art style but focused around a set of brutal murders that happen towards the front end of the story and the survivor's guilt that follows.
Gunnerkrigg Court is hard for me to describe but gets a re-read from me every few years. A surreal examination of why we tell stories and The Power Of Friendship? It's been going so long that I half suspect it's not going anywhere and is just an excuse to keep chugging for as many years as its author needs it to, and also half suspect it's imminently ending in the next few months. I don't think I really have a problem with it either way; it's written very well.
I also want to add Questionable Content. Someone else mentioned it before but I think it needs linking to. It's one of the first webcomics I picked up so it's hard for me to judge it fairly, but even having evolved so much since the early days I think there's something to love about every stage of its development. It was just easy reading about hipsters listening to music and drinking coffee, and then somewhere along the way it became a sci-fi story too and goodness I love it.