is definitely the best place to start for most things. She has guides for everything from low level flashpoints to the current tier of nightmare ops. Some of the strategies she suggest are not the best way of doing things, but they're all pretty accurate except for a few (which have probably been updated by now). All the mechanics are there though, and she does a good job explaining differences between SM and HM (and Nightmare). The site is also probably the best place to look for guides on server events (Rakghoul plague, Bounty Hunter week, etc), upcoming patches, and new additions to the Cartel Market.
If you're looking for some video perspectives, Drop it Like it's Hoth (DiLiH)
and Death and Taxes (DnT)
both have great videos for 8 man kills (at least on Nightmare, probably a few HM as well). Severity Gaming
is a good place to start for 16 man videos.
Lastly, this forum is a pretty good place to get some advice if you've got questions about specific fights, since many top end raiders frequent it. You can probably search for previous threads that people have made to get some tips on some of the harder fights that guilds have issues with (Dread Guard, TfB, Styrak, etc).
Hope this helps; best of luck!