While I assume you will eventually get this finder for ALL the stuff, the question is MOST important right now.
Then I would say Flashpoints, Story and Hard mode
It's a fast and better way to geared for OPs. I have barely played ANY of the Flashpoints and thats because people just think it's too big a hassle to get a group together. Everybody WANTS a group, but nobody want to MAKE a group. Which is sad and silly, but very true.
I would put Heroic's second for the same reasons.