For me it's because that arc had a lot of story and was set up well. It had enough character progression but wasn't overwhelming. It also introduced a lot of mystery i.e. bankai's and why ss was being so weird. I think it was good to have that organization as a villain. it also set up aizen which was done very well and only got dumb when aizen combed his hair back.well , in all honestly , i don't see what's so great about the soul society arc fella.