I've regretted most vehicles that I let go of. Most recently a built '94 Trans Am that I traded for an '85 Silverado 454, but I've always wanted and old squarebody. I miss letting go of my '10 Camaro SS too. Both GNs are forever in my happy memories, but the first the most.
True regret has been every time I got rid of a truck for a car. Always a stupid idea to not own a truck.