It does depend on what you like.
If you like thrill rides then Kraken and JTA are definates. The simulator part of Wild Arctic is one of the better types of that genre.
The shows of Shamu, whale and Dolphin plus Clyde and Seamore are all entertaining. If your thinking about visiting cirque du Soleil ( at DTD) then a visit to Cirque de la Mer will give you a very small (but FREE

) taste of some of the type of acts you would see at the costly but wonderful Cirque de Soleil.
On a conservation note the Manatee exibit has probably the strongest message attached to it and seeing these creatures gently paddling around from the underwater veiwing area does make you reflective about what could and should be done to protect them.
If you have kids, feeding ( and hopefully petting) the stingrays and dolphins is always popular, as is feeding the seals and sealions ( but I wouldn't advise trying to pet these

). I could watch the penguins and puffins for ages as they always have the ability to amuse and the walk through/animal parts of wild Arctic are very impressive ( and a nice chance to escape from theFlorida heat.
#1 must do for me though is to make it to the hospitality area to avail myself of a couple of free samples of the wares of Anhauser Busch

.
To cover all of SW in one day is not easy, if your schedule can take it and they are offering 2nd day free ( pretty much on offer all year round) I would suggest taking the opportunity to have one day an early start and cover as much of the park as you feel comfortable in ( taking in one ofthe early Shamu shows) then come back on your second day a little later and stay till closing. This gives you the chance to take in the evening "Shamu rocks America" show which is a different pace and atmophere to the "regular" day shows as well as the impressive evening fireworks and laser show that SeaWorld closes the park with.