Depends on budget and type of car ofcourse, but taking my own Boc as example I would think these really change the car for the better;
Turbo back exhast - Just makes your driving experience better, makes the car sounds alive. + also good for performance.
ECU tuning - getting a proper map on a dyno will give more power and sometimes better overal drivability instead of the factory treehugger map. combined with the exhaust it will put a smile on your face.
Proper lower set - be it springs(and dampers)/coilvers, a proper lowered car will improve handling, gives the car a better stance, and will remove some of the "old people" compromises that the VAG concern dumps in almost all of their sporty vehicles
For me these are the must haves on any car I've owned, exhaust, ecu, proper lowering set. Because those three things just improve (for me) the driving experience and removes the "everyone should be able to drive our car" compromises the factory has put in.
Whiteline rear arb, upgrades (brake)pads are also a big improvement if you drive fast. +add brake fluid if you do track days.
If you dont like the stock wheels, upgrade them. Matter of taste ofcourse, I have changed the stock wheels on every car I owned, so for me this is almost an essential.
Last but not least, a spoiler or a skirt here and there.