Seminyak is by far the nicest, in terms of restaurants, shops, beach and villa/hotel quality. I stayed at Nusa Dua on my first trip to Bali and hated it -- a 5-star butlered desert that could be anywhere in the world except Bali. Ubud is cooler and rainier at times, also much smaller and now very touristified (but still charming). Sanur and Kuta are cheaper and it shows, ok if you know your way around but why bother?