Oohhhh I really want to guess which comic that was.
I know the Sam and Fuzzy forums were very lively but I'd rank that in a higher tier.
QC might qualify these days but my past self thought it was the best so I'll rule that out out of respect for younger me.
LFG was kinda like a lesser Nuclear Power but I've got no sense of their forums. The timeline's off there too, given the ballpark I'm assuming from your comment.
Dragon Tails was solid upper middle, also had a very healthy community forum.
Bob and George was definitely lower tier but in a fun way.
What was the one with the pink cat? Ahhh, Something Positive. I remember thinking that was great, but I don't really remember anything else about it.
Going further back, into possibly pre-burning-boards territory, there's what... El Goonish Shive and Unlike Minerva. I'm not sure how well-known Unlike Minerva was, but apparently EGS is still going strong.
Around there was College Roomies from Hell which opened me up to the whole Keenspot family, and there are a lot of directions that can go.
Ruling out XKCD since even if you don't like it, you have to acknowledge how widely loved or at least known it is. Probably same with Penny Arcade.

Aaaah, the list on the Shortpacked landing page is full of nostalgia.

I won't accept Mac Hall as being classified as anything less than top tier.

I think I'm gonna have to go with Real Life. Nice, comfortable, consistent. Cozy. Not universally known but a good solid regular webcomic. Has (oh, had) forums, fits the arbitrary timeline I have in my head.
Whaaaa, Mae Dean has certainly gone through some changes since I checked in last!

I might change my answer as I continue down memory lane here.