My most favorite has to be Cobb tune. I love the power and practicality. The second best would be the 19" staggered M6 reps with larger front and rear foot print. The third would be a tie between my RennArt catback exhaust and BMW M performance racing alacantra with red/blue stitiching flat bottom steering wheel with M3 retrofit paddles with + for upshifts and - for down shifts. The steering wheel feels thick and soft perfect for spirited driving.....and sound of RennArt exhaust is nice and aggressive without become fart can nasty.
Next up on my list
might be down pipes and M3 front and back complete control arms.