Definitely tires.

I almost never have to deal with ice, so I don't use studded tires. For me, it's tires that provide good puncture protection and grip but still feel lively. The difference between good tires and bad tires makes an enormous impact on how much I enjoy the ride, even if they aren't going flat. I've had several tires I liked a lot and several that were not so fun. I just got some Conti GP 4 Seasons that I'm loving right now.