for celtics, i often go to reddit. there are some very good posters, but the format is really quite difficult to read.
i also post on SonsofSamHorn. there is not better site for all things red sox. its posts usually are the best, most informed, filled with thoughtful insights, LOTS of homework, and marked by a general atmosphere of overt masculinity. they dont hesitate to let posters know if they did a poor job.
be forewarned, in general the posters tend to be extremely well prepared and they suffer fools very, very poorly. bring your best work or dont post. i see folks regularly banned for poor posting.
membership is by invitation only, though they accept applications for "lurkers" as well.
they also have celtics threads which are also very well researched. to compare, the very best here at celticsblog would be fine at SoSH. the rest of our content would be purged in a hurry.