I like to mix it up. Flat is fine, hilly is fine. I'm on a bicycle after all. On the idea that flat is easy I beg to differ - if you believe that you have never ridden 90 miles into a stiff headwind. Hilly country usually means winding roads, so you are not stuck going into that headwind for long stretches at a time. Just like hills, flat country has its own mindset you need to be in to enjoy it. Those who really like to bike enjoy it wherever they happen to be riding at the time.