I've done so much (with so much more to do) that it's nearly impossible to pick just one. Rims/Tires, bumpers, rock rails/lightbar, lights/switches, Satachi grill, blackout hood and valence, seat covers, the list is nearly endless.
But I gotta say...probably my most fun...and thus my favorite is...
My Flippac shell on the back! Since getting it 11 months ago...I've spent over 45 nights loving the outdoors with great nights of sleep out in the back country.