Terry Pratchett's Discworld series - I was a massive fan of these from my early teens, and it's hard to understate the impact that had. The mixing of fantasy and political allegories, the characters being a group of bumbling losers who are going to save the day anyway, the villain being a posh guy with skewed ideas who thinks he's owed power... And of course the tendency to set up something big and fantastical and then juxtapose it with something mundane and anti-climactic as a punchline.
Megatokyo - The first webcomic I ever read, so on some level this set the baseline for what "a webcomic" is. I think also older webcomics like Scary-Go-Round, Gunnerkrigg Court, and Shortpacked! all had an influence on how I approach writing webcomics.
Sweatdrop Studios - These people basically taught me how to make comics, and encouraged me to have my own unique voice as a British manga creator instead of striving and failing to write and draw exactly like Japanese manga. I learned a lot both literally from them on their old forums, and from reading their various comics.
Scott Pilgrim - Massively influenced how I pace comedy and action, and I'd call this the biggest influence on how I write dialogue specifically.
Kiyohiko Azuma - In terms of comedic timing, and somehow making the simplest possible joke absolutely hilarious, this guy is like a god. Absolutely incredible. I strive to be as funny as Azumanga Daioh and Yotsubato!
Homestuck - The Homestuck influence in my writing isn't so obvious in Errant, because I had to cut down the number of words when I saw how little I can fit on pages to make them comfortable to read. The older scripts are much more rambly. I feel like the influence of Homestuck comes up most in Jules' dialogue, and in the pacing of some action and comedy.
William Shakespeare - No, really! I'm actually a massive Shakespeare fan, and I've been to see a bunch of them, including one at the Globe. I love the way Shakespeare builds plots with meticulous mechanics, and then tells them in the clearest possible way. He's always thinking about who needs to be where and how the misunderstanding happens. I love his wordplay and his use of these big concepts and symbolism. There's a lot of quite Shakesperian themes in Errant; a bad king whose folly curses the land, with a sibling who is a wise fool, somebody in a mask, misunderstandings keeping people apart, gender confusion due to cross dressing, fairies and monsters, ghosts etc. etc.