I always go to Lake Sallie for a number of reasons.
1. It's for the most part pretty close to the stage which is nice.
2. There is always something going on there and it is also right next to Northwoods which can be a little more crazy, so if your feeling bored you can walk over there, but still find some peace at night to sleep if you choose to.
3. The big selling point for me is the Lake. It's not the nicest but just going in there in the morning when you're feeling a little hung over and probably a little dirty changes the day for me. You can walk out quite a ways and there's always someone with shampoo and soap to clean up a bit.
I'm sure Northwoods is great also but have never stayed there, I feel committed to Lake Sallie after three years of camping. One recommendation is to buy your camping pass early for one of these two because they do sell out early. Three years ago I got stuck in Viking which was sunny, hot, no lake, and ridiculously far. It took like twenty minutes to walk to the stage and almost ruined my weekend.