I've visited four Sunday's so far this season and it hasn't been too bad until about 2pm or so and then it wasn't real crowded since it was early in the season
. If you get there early (by 9 or so) and wait in the Season Passholder line you'll at least be a little ahead of the game. You might want to decide what you really want to ride first, SUF or Bull and then go to the approriate side of the park to wait at the barrier.
I would expect that since we're past Memorial day and lots of schools are out that it will be more crowded than I encountered in my earlier visits. There were quite a few school groups on my last visit a couple weeks ago but they didn't clog things up too much.
I would ride the heavier demand stuff such as Bull, Viper, SUF, Batman, etc. as much as you can early. As things get busier you'll find a little less of a wait for American Eagle, Iron Wolf, etc. .
No one can ever predict what's going to happen with crowds on a given day except the park usually has some idea based on group pre-sales what's going to happen. Occasionally, I've struck up a conversation with an employee and they'll comment that there XXXX number of pre-sales today.