Fountains of Bellagio = Vegas
This is a must see for all visitors. Best at night where starting at 8pm the shows run every 15min so it's easy to catch two. Tip, even when it's crazy busy, most clear out moments after a show. This is the best time to get a prime spot at the center of the fountains and wait a brief 10min before the next show. My favorite shows are Con Te Partiro and Ecstasy of Gold. Con Te Partiro is clearly one of their best because it is always the first show of the day. Other good shows are Tiesto (the longest show I believe) and UpTown.
Show calendar changes out every few months. Attached is the schedule for December and it's expected to change in early January.