17's over 18's.
16's over 17's.
17's vs. 18's:
17's will be faster, brake better, handle better, not get damaged as easy, cheaper, cheaper to buy tires for, not so ridiculous looking, will not rub as much.
18's look tacky, especially with all of that clearance between the brake assembly and the wheel. If you're gonna go 18's go 18's for a reason...get the biggest F'in brakes you can find.
16's are superior to either because of even better accel, brake, cornering (depending on your tire choice) etc etc.