My vote would be for Jekyll Island.
The short answer, any of the three would be great. The regulars here on this forum have their favorites and it's pretty much split.
Maybe write more about what you are looking for and we can better point you to the "best" island for you.
We like beach at Jekyll but hotels, restaurants , etc at St Simons.
Amelia Island is nice too.
Amelia has a neat historic downtown area and a Hampton Inn right there. Plenty of neat eating places in that area. Not as much to see on Jekyll. St Simons not as "beachy".
Our family loves Amelia (we've been to Jekyll but not SSI).
As Stevedec noted the historic downtown Fernandina Beach is full of charm, good restaurants, unique shops and galleries and home the oldest continuously operating bar in Florida (Palace saloon). Nearby Fort Clinch State Park and Egans Creek greenway offer nice hikes, bike rides and nature.
Cumberland Island National Seashore (in Georgia) is a nice day trip from Amelia.
Is the ferry running again to Cumberland? Lang's docks really got torn up during Irma.Edited: 26 September 2017, 11:36
I don't think so yet. It would be worth a check of the NPS website.
I read that the deductible on each of his two docks was $100,000. That's got to hurt. Not just the huge destruction but the huge cost to rebuild. I don't know what I'd do under the same circumstances.
But Cumberland IS wonderful! One day I'm staying at the Greyfield Inn!