Obviously a nice bike is subjective. I think it starts with what you started off with or your needs were in the beginning of your cycling journey. For me it meant a bike I could train on and race on. Since my racing was triathlons that usually meant a road bike with italian geometry and aero components and it still does. But a nice bike still needs to be solid no rattles, nimble enough to take a tight turn and stable enough t hold your line in a pack. When you find one buy five more, that's what I did.