Well, if you have played Ofp, you already know 70% of all you need. Lots of things have improved, but the basic eskeleton is the same.
I liked the British addon. Nice recopilation of new soldiers, weapons, vehicles, 4 mission campaign, and a 9 extra sp and mp short missions. Buy it now or never, as it’s also with the 50% off.
First play the boot camp, the second set of missions, from E01 to E15. Play the other set after, if you want, but it’s the same but not as well explained.
After, the single player missions (not campaign). From 01 to 04, and from E01 to E07 (the E missions are from Operation Arrowhead).
You can try a bit the Armory or the rest of sp missions. Or the showcases to see all the hardware.
Then, play the campaigns. You have Arma 2 (which try to put a bit of rpg and open adventure in the game, but fails around the end), OA (more varied and better polished, but less ambitious and with less focus on plot), two missions from a pseudo official campaign called EW (pretty well done), BAF which like OA is polished (for Bohemia :P) and very fun gameplay wise, but kind of without character.
From the user made missions, i am trying to remember names without a lot of success. Search Black Forest mission and others of the same author, and the bardosy campaigns. There were a few ones more which surprised me, but i can’t remember more names.
The ultra realism mod is called ACE, if you are into that kind of thing.
There was a good sound mod called JRSS or something like that.