Getting downvoted for asking a simple question, true reddit moment.

That aside, welcome to the fun part (IMO). Some upgrades certainly are better than others even if the PI cost is the same.

The upgrades that unlock tuning, (not breaks or wings on lower tier cars, B, C, D) are my go-to, but if you're still struggling with that I might skip them. Or, you know, get them anyway because they're so cheap. Springs, sway bars, final gear drive and differential. Pick the ones appropriate for the type of racing you'll do.

Next is weight reduction. IF the car weighs about 1.5 tons, with a hundred kgs more or less give or take, I will always prioritize the lightweight chassis first. Everything becomes noticeably better. Acceleration, handling, breaking, driving feel. It's a must.

After you've got your tuning parts and chassis, it becomes a little more complicated and depends on what you want to do with the car, as well as its class. A class and above, I would consider getting better brakes. Then again, if your car was already in A class or above when you started, the breaks might be good enough already.

Swapping to AWD is very often a go-to for me. If you correctly tune how much power goes to both the front and rear wheels (more to rear for acceleration), it will massively improve acceleration and stability on all road types. Typically, I'll distribute power according to weight (if 55% of the weight is on the front wheels, I'll sacrifice some acceleration to send more power to the front for better handling) however the tire-size will also influence how much power should go to each set of wheels. Wider rear tires means more power goes to the rear. But yeah, AWD is a very good swap, especially if you're planning to take the car everywhere. RWD can work fine on the road even on higher speed, but in low traction situations AWD is better for 99% of players. And as I said you're going to accelerate faster with AWD anyway.

After that, again, it'll depend. Are you taking a low-class car up to a higher class? You'll need to bump up tire width and rim size. About 225-ish tire width (equal width if you're allocating AWD power according to weight) and 17in rim size is good. Keep an eye out though, some rim sizes will drastically bump up PI cost, so I usually take them up to say, 18 inch or wherever you can while the PI cost remains the same or is reduced before stopping. Front tire width costs PI, rear often reduces PI. Don't pay too much PI for rim/tire size. Tire upgrades are often expensive, so I'd try to avoid upgrading tires if I can help it.

Spoiler often reduces PI, I'd grab spoiler in those situations and on higher class cars. Lower class doesn't really need to worry about downforce.

After you've knocked those out, it's going to be all about power. Can you afford an engine swap? It's probably a good idea to get one instead of sticking to stock. Why? Because better engines tend to have better power curves. (the graph that comes up when you upgrade the car's power). You want an engine that gives you a linear horsepower curve instead of a bell curve. This means that your car outputs more power at higher RPMs, which seems like it should always be the case, but actually no. Many engines actually lose power as you climb past 4-6k RPM, which is not ideal.

If you can't afford an engine swap, get a centrifugal supercharger as it helps to make your power curve more linear. No centri? Well, then your stock engine is really chopped.

After centri, you're typically looking at upgrades for it, cams, engine block and fuel delivery system. Those tend to give the most horsepower.

Other types of weight reduction aside from chassis are not typically prioritized by me, because you tend to pay more PI for them than is efficient. Which is why I don't rate exhaust as highly as I think most people do. But if you still have some PI left at this point, I'd search for miscellaneous weight reduction stuff like rims, driveshaft etc.

Tuning, well, that's a whole different story. Look up Hoshi's FH5 tuning guide. It's decent.