The most important ones: Subframes and gears. I like 3.73s cause im on the highway alot and the car can light them till 90mph. Alot of people on here like 4.10a

Off road H-pipe is a good gain and catbacks make the cars sound

They all have a distinct sound so choosing cat-backs is purley what YOU like, dont fall into the hype. Flowmasters are not the best but they are cheep and I like the sound so I have them. Long tubes are nice but very expensive to buy and have installed. K&N filter is just fine with the stock box. A JLT or WMR intake do pffer a gain and look nice but you will loose the strut tower brace. Wheels are important cause you can fit alot of tire under our cars, just not on the stock wheel
Because of the cars age I would do: Springs/shocks/struts ( I like Eibach), Inner tie rods (get good ones), Steeda Bumpsteer kit, CC plates, subframes, Rear LCAs and check all the bushings. The car will feel 1000X better. Trust me I just did this last week and the car is spectacular to drive now

Also cause I just went through this, if you do the brakes make sure you re-use the old anti rattle clips cause most new pads wont come with them and without them the rattle is
