For wrecks I'd definitely recommend Istrian Peninsula. The most famous wrecks are the Baron Gautsch, the Hans Schmidt, the Coreolanus, the Guiseppe Dezza... I don't think it's easy to say which is the "best", depends on the visibility etc. And not all local dive centers dive all of these. Usually only the Baron Gautsch is considered as a "must dive". Here you can find a nice list of the wrecks in Northern Croatia:
Wreck diving - Diving Center Shark - Diving in Croatia
Another good destination is Vis island with a few famous wrecks, but some of them can be dived only for technical divers. If somebody travels to Krk island, can dive the Peltastis wreck at Silo and Lina wreck at the neghbouring Cres island. There are some other nice wrecks like the Tihany at Mali Losinj or the Francesca di Rimini at Kaprije, but there aren't other wrecks, and these ones aren't enough for a wreck trip in my opinion. For wreck enthusiasts I'd suggest a trip to Istria, for technical divers Vis island.
There is a really nice website about Croatian diving with a detailed map of dive sites:
DIVING IN CROATIA
---------- Post added May 15th, 2013 at 06:41 AM ----------
Nice to hear! I hope Anna and Kurt organized some nice dives for you. Yes, sometimes there are more plankton in the Adriatic in May, but you never know what to expect.
Korfu? I've been to there, but for me it's very different than Croatia as I wouldn't go there by car. There aren't much fish in the sea and it isn't warmer than Dalmatia, so only for diving I know better places in Croatia. (Anyway, if you go there, stay in Paleokastritsa town, the best dives sites are on the rocky western shore.) But Greece is nice, and as I'm a fan of Gerald Durrell's books I had a nice time there- with a few average dives. There was only one really memorable dive site, a cave:
A cave to remember - divingiscool.com
Krk and Cres diving is very similar, simply because from Krk they dive often the walls of Cres island. The most famous wreck is the Lina at Cres what they dive from Krk too. Krk island is a lively place with some nice towns and villages, I spent nice holidays there, and I know some dive centers. In Krk I did daily 2 tank dive trips and some nice 1 tank dives as well, I usually don't prefer spending the whole day on a boat. I don't know too much about Losinj, I think it's must be similar to the other Kvarner dive sites.
I think diving at Rab island is more similar to Pag, although I've been there only for a short trip. I know more about Pag, there is a village called Stara Novalja where half dozen dive centers work: they dive the opposite walls with fast RIB's, and some of them has nice house reefs as well with seahorses. I'd go here for some nice and affordable Adriatic diving experience, but it's a small village, there are much less bars, restaurants, etc. In the beginning of June we will go there for a special UW photo trip, because the location is ideal for that and we are not interested that much in sightseeing this time.
Mljet, Lastovo? In the past the adventorous divers went there but they are so far away! And the experts say you won't see more than in the islands of Southern Dalmatia like Vis, Brac, Korcula. So I confess I had only plans too, but never traveled to these remote islands. I had some really nice dives in the Rogoznica area where the water was always warmer too. Oh yes, another idea for nice wall diving experience: Dugi Otok. They dive sometimes the Kornati as the centers in Murter or Biograd, but the walls in the western side of Dugi Otok as nice as well, and I had my best diving experience there during a liveaboard trip. There was a reef called Kampanel Misnjak with colorful sponges, huge crabs and scorpionfish, morays, etc.
I know some dive centers so if you would like to go somewhere, maybe I can recommend a reliable one.
. These guys were unbelievable, but the dive master told me that is a common problem. 
. The vis was so-so, lots of plankton. They have nicely grown walls, sea fans and lots of fish (saw a moray about 1m long!)
