I like all of my cars as stock as possible, with "realistic" tires. I enjoy my cars that way much better.
Things that I do to my cars:
-Window weight reduction
-Weight reduction 1 (Sometimes. Not always)
-ECU chip (depending on the car)
-Sports Cat
-Exhaust manifold
-Sport Exhaust (ONLY if it fixes an incorrect sound sample)
-LSD (Sometimes. Mostly in old RWD cars)
-Turbo and Supercharger. (Very rarely and only if necessary).
-Comfort Soft Tires are a must below 320HP Eveything else; Sport Hards.
-Suspension must ALWAYS be stock in my cars.
-Transmission (only if necessary).
And that's pretty much it. I might play around with some parts with some cars, and also experiment with brake bias and ballast, but usually I get bored and return them as stock as possible.
Personally I like to feel the differences between each car, and feel how the car was meant to be when it came out of the manufacturer. I don't know why... it's just me. I don't like to tune everything to the max to the point were the real car is no more. I find no point on that. But again, that's my personal opinion.