Vattu, Rice Boy, KSBD are ones that hooked me basically right off the bat. Then you have something like Homestuck which I've found to be a little boring, but am slogging through because I think I should read it.
Generally speaking, I really like the narrative-driven comics with a lot of world building (which is what I hope I can accomplish) and the rest don't grab me so much.
Vattu's tenth page, they're throwing a freakin' baby over an open flame, so that's kind of awesome.
By Rice Boy's tenth page, we have a noir robot prophet trying to convince a small mushroom child he is the chosen one, so that's p interesting.
KSBD? By the tenth page, a normal college girl has a magical key stabbed into her forehead by a dead god and is transported to hell. So. Ya.
By page ten I'm either all-in or bored (although, I will continue reading if bored to give the author the benefit of the doubt, to page 30/40/50). And then of course you have the comic strips like Pigeon Gazette, Owlturd, and Extra Fabulous Comics--I always appreciated one when I found it, but I never checked out their primary hosting sites until I'd seen dozens and dozens each, respectively.
*Also, I'm a sucker for good action. As a huge fan of Genndy Tartakovsky, I LOVE to see highly stylized fights--especially when they involve a large number of participants.
That said, I know that sometimes my tastes and habits don't line up with the common ones, so that's why I'm asking! AND, ofc, I too have never really stumbled upon a NEW webcomic, so I'm not sure what I would do if I literally found one day one that only had 10 pages ready.