I've been checking out tires for both my 3.6R (17" tires) and my wife's Toyota RAV (V6).
I've found a tire that fits both vehicles and has pretty high ratings for all seasons:
Goodyear Assurance ComforTred Touring(Grand Touring All-Season)
Not sure if they will have an 18" size for the GT wheels.
Otherwise, the next best tire (in my humble opinion) is this:
Continental ExtremeContact DWS(Ultra High Performance All-Season)