I've never seen (read?) one. Which one is the best? Are they visual, text, what?
Most web series arrive in installments of 3-10 minutes each.
The ones I'm thinking of are video, although of course other formats exist.
My personal favorite is Red vs Blue, which is a comedy series created in the "Machinima" style (basically, using a game engine for easy 3D with voice acting). It ran for 5 seasons as
Red vs Blue: The Blood Gulch Chronicles, and has since produce two seasons of what is being called the "Re-" trilogy,
RvB: Reconstruction and
RvB: Recreation. (The working title for the upcoming season is
RvB: Resolution.) Each season runs approximately 2 hours long on DVD, although the actual length is variable. RvB is sponsored by Microsoft. While Blood Gulch Chronicles was great, there's no question in my mind that
Reconstruction is the best thing they've done so far, so that might be a good
starting point if you wanted to take a look at it.
Some live action web series I've seen include
The Guild (funny, but I've seen better) which is about a group of MMO players, and
The Legend of Neil, which is a bit on the offensive side but still manages to be hilarious most of the time. The Guild got sponsorship by someone or other, and I think The Legend of Neil is officially a Comedy Central show, although I can't imagine it actually airing on TV.