Discussions like that just show me how much I still don't know - I really don't look to much on the studios usually, but here are some I like:
OBASAN: Lots of good milf videos - always have an eye on their newest releases.
FITCH and Wanz: I like their videos in general - good stuff and good variety for all kinds of interests.
MS Video Group: Good for all kind of the more dirty stuff I like (e.g. cum eating videos)
Since An Tsujimoto's and Riku Minato's switch to S1 I also watch their newest releases closely, although I don't like their videos too much, but for these two actresses I need to make an exception.
I also like Rocket for more crazy videos, but just from time to time.
But there seem to be a lot of good studios, which have at least some very interesting series.