As it is your first trip and perhaps you don't know for sure whether you will get back there again or perhaps not want to go back again (foolish notion I know), I would go to all the main theme parks ie the Disney ones, IoA, SW and US. Each and every one of them has something different to offer. It would be a crying shame not to sample them all on a first visit. We still visit all of them.
As regards Epcot, I like the park especially Soaring and Test Track. It is a good park to do in two halves. One half first thing and get the rides done a few times and the other half later in the day and wander round the worlds, topping off with an unbelieveable firework display.
Last year we went in July which is a busy time but lines are not a problem if you hit the parks early. As a result we didn't want to waste a morning by going to a water park first thing preferring to have more go's on rides. Instead if we were in the Disney area we hit a water park after the morning theme park, late in the afternoon and had no problem getting rings in the creek or going on rides. Just a thought.
Also if you look at the askdaisy.net website for the Brit Guide to Orlando there is an update on the quietest days to visit parks. I would also recommend avoiding Disney parks on EMH days, they are absolutely heaving and much better visited the day after. A lot of people don't buy hopper passes, I believe, so have to spend a day in one park. It, therefore, makes sense if you staying on site to use the EMH to get more money out of your ticket but everyone does this.
Usually we do Animal Kingdom the first day, only because it is the quiestest day when we arrive. We do have a little race round in the morning getting the rides done but then it is so relaxing just wandering around seeing the animals and shows for the rest of the day. After a long day travelling the previous day it is nice to choose a park where you are not having to line up all of the time and can just wander round. Again Epcot is good for this too because it only has a couple of rides that attract longer lines most of the time you can just wander round viewing things. Also Sea World is a relaxing park too.