16's unless you absolutely must have wider than a 7.5 or 8" wide rim and 225 tire. If you must have 235-255 and want to squeeze in 8-9" rims, 17 is the way to go.
Any tire less than about 45-50 profile (You can get away with 40 on wider tires in our size range) will be too stiff to provide good grip off of smooth a race track - you won't be able to maintain the full contact patch on your average mountain road.
Actually, sidewall height is what matters - you want at least 100mm of meat between the rim and the road. 100-115mm is about the sweet spot for a street car in our weight range. Lighter cars can get away with a little less, heavier cars need a bit more.
As far as appearance goes, 16's look appropriate for the car, 17's look a bit more aggressive. 18's simply look silly.
Also, 16's with 225/50s are a very economical combo. Jumping up to 17's and 235+, you'll be spending more for wheels, and close to twice as much on tires