Obviously this is subjective. But the first thing I did was sways. I don't touch suspensions otherwise anymore. Too many bad roads and too many miles driving on them with coil-overs, springs, etc.
Second is the oil catch can. I don't have to do any valve cleaning with mine. I just have to remember to empty it. Oil changes are a good time to do it.
Spark plugs do not need to be touched unless you're going to increase boost (JB4, et. al.). And then you want one of two specific
spark plugs mentioned here endlessly.
I did the midpipe and so far nothing more. Stingers were very new and I just looked under the car. It seemed the midpipe had the largest/most bottleneck(s). I still had to heavily insulate the hatch area and under the back
seat to help with the increased exhaust volume. If you do consider replacing/upgrading further back, look into tips that will extend the piping to the decorative outlets in the rear valence. That will keep the exhaust noise from bouncing off of those tips and back to be heard in the car. Search is your friend.
The JB4 is the ticket if you don't want a custom tune. But you HAVE to have good gas. True 93 octane or E30. You'll want intakes,
spark plugs, and whatever connectors they may have for easier installation/removal. Again, plenty on this topic through the site's Search.
Good luck and have fun!!