Riverside is our preferred moderate, though many will argue for French Quarter. Both are connected by a walking path along the river (which I HIGHLY recommend taking at least once, as it's lovely!) so you can easily explore the amenities of both. Coronado is also very nice and I feel the quality of the rooms is slightly above that of the other moderates (ie, wooden doors to separate the bathroom/bedroom, as opposed to a curtain). We stay at Coronado whenever Riverside is booked, but definitely prefer the quiet, laid back ambiance of Riverside. We've not stayed at Caribbean Beach but have walked the resort (we used to live locally, so walking the resorts was something we did for fun) and were not terribly impressed with the amenities. The main issue with Coronado and Caribbean Beach is that both are under heavy construction at the moment. Perhaps someone else can chime in with whether that will be finished by May 2019, as I'm not sure. Hopefully this helps your decision-making!
- Lauren