If you have mk's programme, these questions are answered within it.
The best brakes are generally F1 brakes, along with the Zonda Race car, the TS020 Race car and a couple of others - in the programme there are numbers in brackets following the brake type (like 44/29), this indicates the relative strength of the front/rear brakes - the higher the better.
Brake Balance Controllers are all the same. Besides which, if you're hybriding you can set the number to whatever you feel like in the .ini file.
The "best" chassis are obviously the F1 ones - they are the lightest and most agile. However they may end up looking ridiculous on your car's body, as the wheelbases are often totally different. The best one tends to be the lightest one that will adequately fit your car (allow about a 30mm difference at most).
All flywheels, driveshafts and triple plate clutches are the same.
All power parts have their power multilpier written after them in mk's. Bear in mind that 100% is a power multiplier of 1, so a car with 200hp and a 102% power part will have 204hp, not 404hp.
Grip should be left alone.
Downforce appears to be cyclical - go too high and the value resets to zero (which is fun to drive with). Set a value of zero in the ini file and you get the maximum possible downforce on front and rear, but it may make the car tough to handle in high-speed corners. Go with what you find easiest.
All this, and more, can be found if you click on the link in my signature.
